Finish the main quest. Now, I recommend you start out at Casual. No, Normal is NOT Hardcore, and Veteran is NOT Insane. So, go easy, find out where everything is at. Save your money for the next playthrough. Make sure to always use two allies and do all of the side quests with them.

Feros is the planet that has the colonists and is not covered in snow. Now, all you have to do is: Talk with the leader, kill the Geth, now this is important: Talk with him again, then talk with the people in charge of: Food, water, and power, and do their side quests! After that, move up the bridge, go to ExoGeni, find Lizbeth, return to the colony. Now, either upgrade your grenades with the Thorian gas or kill them all. Now, if you kill them and didn't do the side quests, you will NEVER complete them.

This is your first mission. This is important: If you want Insane or Hardcore, you can change the difficulties in-game, but do it BEFORE landing on Eden Prime! This was confirmed in the Bioware Forums. Now, just kill the Geth then de-activate the bombs by standing near them and holding A. After that, approach the Beacon and you're done.

Virmire is the planet you receive from the Council. First, you land, clear the AA guns and approach the Salarian Camp. You can get the Charm/Intimidate on Wrex, who will want to get the cure for the Genophage. Don't worry if you don't have it maxed out, there's another chance later in the game. Now, choose between Kaidan and Ashley to go with the Salarians. Later in the battle, you will have to choose who to save. IF one dies and you're doing the romantic sub plot, it will end there.

Noveria is the planet covered in Snow, and has Matriach Benezia. In order to get the garage pass you need to help Lorik. Go to the garage, kill the Geth, and drive up. Once in the facility, you'll need to activate the VI's core. Either doing the Tower of Hanoi puzzle [search in the site or google for how to do it] or sacrifice 100 omni gel. Then, activate the power and landlines. Once in the tram, you must kill the Rachni, but first go down the elevator to where the survivors are at, get the pass, and to the hot labs. Activate the Purge and get out. Return to the survivors, (SPOILER: Kill them). Go to the end of the place, and go to the tunnels. Find the Matriarch. To kill her, you MUST kill the Geth and Asari Commandos. DO NOT FIGHT HER. Start off, kill the Asari Command rushing you, then RUN TO THE RIGHT and kill the Geth and Commandos after talking with her, she'll go berserk again. Kill ALL enemies, then kill her.

After finishing your Casual, or any play through, select new career and existing ID. Make sure to change the settings in the main menu, difficulty to Hardcore.

WARNING: Whe you start the game in the Normandy, check the setting again and make sure it is set to Hardcore to get these at the same time! If you land on Eden Prime and it wasn't set right, you will not get the achievement for Hardcore.

ALL organic enemies have immune. They are very accurate and WILL cut you to shreds. Take cover often, but watch out for rockets. Use the Spectre Master Gear and a lot of Biotic powers. Lastly, all enemies are the same level as you.

Refer to the Long Service Medal achievement.

Refer to Distinguished Combat Medal and Long Service Medal. Note: ALL organic enemies have immune. They are very accurate and WILL cut you to shreds. Take cover often, but watch out for rockets. Use the Spectre Master Gear and a lot of Biotic powers. Oh, and for fun, enemies and bosses are higher levels than you.

The pistol is available to all classes and is the easiest one to get if you start working on it right away. No matter your class, put in enough points into the Pistol tree so that you can get the Marksman skill, as it is immensely helpful for this achievement. When you get the skill, use it as often as you can, as it makes using the pistol very effective. Even if you don't play as a Soldier or a class that doesn't have any training for that particular weapon, it is still very possible to get the achievement. Focus on getting as many accuracy improving attachments for the guns you aren't trained in, and stabilizers for Sniper Rifles. It's not as good as playing as a class trained for the weapon, but it's better than nothing. Also, kills made by teammates do not count towards this achievement.

Note: Sentinels don't have pistol training. They get pistol bonuses from their class skill.

All weapon kills carry over between playthroughs and careers. Even if you don’t get all 150 kills in one playthrough, any kills you have made will carry over into every other one you may have.

See Pistol Expert for more info.

The Shotgun training is only available to Soldiers and Vanguards, but can be done by anyone. Focus on getting as close to enemies as you can, fighting Husks and Thorian Creepers are the best enemies for this achievement, as they need to get in close in order to damage you.

See Pistol Expert for more info.

Only Soldiers can train to use Assault Rifles, but some of the better ones are viable for use by other classes. Anyone but the Soldier is going to need to focus on improving accuracy if they want to hit enemies, but you must also worry about it overheating. Assault Rifles can overheat very quickly if you don't pay attention, so always make sure it has time to cool down so you don't have to wait.

See Pistol Expert for more info.

Soldiers and Infiltrators are the only class that can train in Sniper Rifles, but with enough upgrades, it can become easier for other classes to use them. No matter the class, focus on getting as many stabilizer upgrades for your sniper rifle, so that you aren't moving around much while aiming down the scope. It's still possible for it to stray with upgrades, so you may have to learn how to aim with the movement. It can be a pain, but it is manageable.

Create an Adept character, as they can use all biotic powers. Go to the Citadel and target a Citadel Rapid Transport station. Use, Lift, Warp, Singularity, Stasis, and Barrier (although you could just run around casting barrier). Then wait for all your powers to recharge and repeat until you get all the achievements. It helps to have adrenaline rush as it resets all your cooldowns, so you can get an extra cast in whenever it is off cooldown.

Note: Ally power usage will not contribute to these achievements.

See Lift Mastery for more info.

See Lift Mastery for more info.

See Lift Mastery for more info.

See Lift Mastery for more info.

See Lift Mastery for more info.

For these achievements, you'll need to be an engineer class character. To use these: Press RB and highlight the power you want, then press A. You can map it by pressing X, then all you have to do is hit RB to use the power. Only do that if you're working on one at a time.

To make this faster, use the manual cast for every power you have, then use adrenaline rush. You'll get a second use of every power with that. Now everything will recharge, rinse and repeat.

YOU must do these, ally casts don't count.

Must be an engineer.

The only one of these achievements that MUST be done on an enemy. First, level up to a level 10 and then use all your points on the AI Hacking power. Then you can hack any of the enemies you come across. You will know when an enemy is hacked cause it will attack other foes of yours and will indicate to you it is hacked instead of having an energy bar.

Just find a turret to use it on... Use it, hide, recharge, rinse and repeat. People have recommended the fight with Fist as a great time to do this with the two turrets in his office. Make a save just before this point, hack them both and reload the save. The hacks will go toward the achievement even though you didn't save your game after doing them.

Refer to Damping Specialist.

Refer to Damping Specialist.

For this achievement, all you will need to do is press the Y button. Make sure you're damaged, or else it won't allow you to use the medi-gel. This carries over play any through. It is likely you'll get it after finishing Hard Core.

Refer to Damping Specialist.

This is a missable achievement, but can be done fairly early on, so it's not hard to do in the case you do miss your chance. For this achievement, you must find all codex entries for every race in the game. Getting each codex entry involves investigating certain people while talking to them, which will always be the left option in the conversation circle. Several codex entries can be gained in multiple ways, while others you will only have once chance to get, which is why this achievement is missable. To get each codex entry, you must do as follows:

Aliens: Council Races
Asari: Automatically unlocked after first meeting with the Citadel Council about Saren; Talk to Saphyria. She can be found by the embassies, as she is the receptionist. 
Salarian: Automatically unlocked after first meeting with the Citadel Council about Saren. 
Turian: Automatically unlocked after the first conversation in the game with Captain Anderson and Nihlus.

Aliens: Extinct Races
Protheans (Missable Codex): During the first conversation with Captain Anderson and Nihlus, investigate when given the chance, and Protheans should be one of the choices; After becoming a Spectre, you will be forced to talk with Ambassador Udina and Captain Anderson. After Udina leaves, ask Anderson more about Noveria. 
Rachni: On the Citadel Presidium, go to the Avina terminal by the giant Krogan monument. Talk with her, and investigate and ask about the Rachni; On Noveria, you can get the codex from two people. The first is Han Olar, the volus scientist. The other is Yaroslev Tartakovsky, who is the last person you must talk to on Noveria. 

Aliens: Non-Council Races
Batarian: After officially becoming a Spectre, ask Captain Anderson about his previous mission, while in front of the Normandy. You can also ask him about it anytime you return to the Citadel. He will be in Udina's office at the embassies, on the Presidium; if you chose either the Spacer or Colonist for your Pre-Service history, you will gain a personal quest regarding those backgrounds. Either of these quests will also give you the codex for doing them. 
Elcor: At the embassies on the Citadel Presidium, talk to Calyn, and ask him about the Elcor; On the Rift Station in Noveria, talk to Petozi. He'll be one of the only Elcor you see on Noveria, so he's not hard to find. 
Geth: Investigate with Ashley Williams when you first meet her on Eden Prime; Once you get back onto the Normandy after finishing Eden Prime, ask Captain Anderson about it.
Hanar: On the Citadel Emporium on the Presidium, talk to the Hanar merchant named Delan. Ask him about his speech for the codex. On the other side of the Presidium, talk to the Hanar preaching about the Enkindlers.
Keepers: By the elevator on the Presidium that leads up to where the council is, there is an Avina terminal and a Keeper nearby. Ask Avina about the Keepers; Talk to the Salarian named Chorban, and accept his quest. He will be scanning a keeper, right in your path when you go to meet the council, so it's very hard to miss him.
Krogan: Progress through the main story normally. It will unlock when you gain Wrex as a party member. 
Quarian: Progress through the main story normally. It will unlock when you gain Tali as a party member. 
Volus: In the embassies on the Citadel Presidium, in the same office as the Elcor ambassador. Talk to a Volus named Din Korlack and ask him about his race; Talk to the Avina terminal just outside of the embassies, and ask her about the Volus.

By majority of the game, they mean an undefined amount of the game. I recommend that you choose two characters and go with them till the end. Try to not start a side quest without the characters you want. There around 70 side quests including the main quest ones. So, make sure you do around 60 of them before Ilos. IF you haven't gotten the achievement after that, then you should try exploring the galaxy and doing more quests. Remember to talk with allies in order to generate more side quests.

NOTE! Completing the DLC makes a noticeable contribution toward this achievement. You should be able to earn all of the ally achievements before you get to Ilos if you complete the DLC.

The following is a list of missions in the game:

MAIN QUEST:
Prologue: Find the Beacon

Citadel: Expose Saren
Citadel: Garrus
Citadel: Wrex

Feros: Save the colony

Noveria: Matriarch Benezia
Noveria: Lorik Qui'n
Noveria: Peak 15
Noveria: Reactor Repair
Noveria: Reconnect Landlines
Noveria: Contamination
Noveria: The Hot Labs
Noveria: Rift Station

Find Liara T'Soni
Ilos: Find the Conduit
Race Against Time

SIDE QUESTS:
Citadel: Scan the Keepers
Citadel: Presidium Prophet
Citadel: Reporter's Request
Citadel: The Fan
Citadel: Rita's Sister
Citadel: Signal Tracking
Citadel: Jahleed and Chorban
Citadel: Homecoming
Citadel: The Fourth Estate
Citadel: Family Matter
Citadel: Asari Consort
Citadel: Old Friends
Citadel: Xeltan Convinced
Citadel: Planting a Bug
Citadel: Schells and his Data
Citadel: Negotiator's Request

UNC: Missing Survey Team
UNC: Rogue VI
UNC: Hostage
UNC: Major Kyle
UNC: Distress Call
UNC: Missing Marines
UNC: Privateers
UNC: Hostile Takeover
UNC: Matriach Writings [10 needed, I collected 15]
UNC: Missing Scientist
UNC: Turian Emblems [13 needed, I collected 17]
UNC: Minerals [ Rare Earths: 20, I found 23. Gases: 6, I found 10. Heavy Metals: 20, I found 25. Light Metals: 20, I found 22]
UNC: Signs of Battle [10 Medallions, I found 10] [ID Tags: 3, I found 4]
UNC: Negotiations
UNC: Cerberus[SHOULD BE NOTED: There is ONE other side quest to do with them on the Styx Theta Cluster, does NOT appear on Journal, search for abandoned ship].
UNC: Prothean Discs[7 needed, I found 10]
UNC: Lost Freighter
UNC: Espionage Probe
UNC: Hade's Dogs
UNC: Lost Module
UNC: Geth Incursions
UNC: ExoGeni Facility
UNC: Colonists
UNC: Derelict Freighter
UNC: Listening Post Alpha
UNC: Listening Post Theta
UNC: Depot Sigma-23

Virmire: Wrex
Virmire: Kirrahe's Team

NOTE: DO ALL FEROS BEFORE LEAVING TO THE SKY BRIDGE!!!

Feros: Recover Data
Feros: Power Cells
Feros: Varren Meat
Feros: Feth in the Tunels
Feros: Water Restoration

Noveria: Espionage
Asari Diplomacy

ALLIES:
Garrus: Dr Saleon
Wrex: Family Armor
Tali's Pilgrimage

Simply complete the game with YOU having more shield damage than health damage. Take cover often. This is easiest with a soldier class since they can wear heavy armor. Get a good armor with 4 bars of shield early and this will be simple. Also try to get it on your first play as you will take more damage during your Hardcore/Insane runs.

A very easy achievement, especially if you plan on getting any of the achievements requiring you to beat the majority of the game. A large amount of the side-quests in the game require you to go to uncharted planets, so you will get this achievement early on. As soon as you land on any uncharted planet, the achievement will unlock. You don't even have to do anything, just land in the Mako.

Note: Therum, the planet you will find Liara on, does not count towards this achievement. The planet you land on must be optional for the story.

This achievement is much easier than it sounds, as money is very abundant in this game. Go through everything you can find pick up as many items as you can, and sell whatever you're not using. Also, doing side-quests also gives you money, so do as many as you can as well.

When you get this achievement, you gain access to the Spectre weapons, available in the C-Sec surplus store. These are the best weapons in the game, so be sure to buy as many as you can.

Add the Shredder bullet upgrade to your weapon and allied weapons. Kill anything that is NOT Geth/mechanical. You'll get it eventually. Yes, they carry over to new games. Do side quests. The Cerberus missions have a lot of mercenaries.

Add the Tungsten upgrade to your weapon. By Synthetic, they mean Geth or turrets or anything mechanical. If you don't get this by Virmire, don't worry. The last mission has A LOT of enemies. Do side quests, there are plenty of Geth Ambushes in the beacons.

See completionist achievement for extra info. This achievement is for Ashley.

Do A LOT of side quests. Don't worry if you get one ally first and later the other.  You can get two per playthrough only. It is possible to get these achievements even before going to Ilos if you complete every possible side quest and all main missions. Bring Down the Sky DLC will also help.

Refer to this thread for some insight to these achievements.

Basic plan of attack for ally achievements:

• Complete Eden Prime
• Go to Citadel, talk to Council
• Find Harkin (Chora's Den), Garrus (Med Clinic) and Wrex (C-Sec)
• Take care of Fist (do not pick up his OSD!)
• Rescue Tali in the Wards Access
• Talk to Council, become Spectre
• Head to Therum and rescue Liara

From here, you will have all the allies. Pick the two you want to use and use them for EVERYTHING. If you don't have an ally with decryption and need them to unlock a quest by hacking a computer, switch them in to receive the quest, then immediately go back and switch them out.

Now with your two allies:

• Complete any possible Citadel assignment
• Complete any random space assignments (check this list and see which ones can be acquired simply by visiting a system or planet)
• Complete Bring Down the Sky missions (400MSP off the marketplace)
• Complete Noveria (do all assignments BEFORE leaving the arrival site)
• Complete Feros (do all assignments BEFORE leaving the arrival site)
• Go back to the Citadel and complete any assignments you started on Noveria/Feros
• Complete Virmire (do not kill Wrex!)
• Back to the Citadel once more

You should have the achievements by now, but if you don't, head to Ilos and finish out the game. Many people got a few of the achievements (mainly Liara) at the very last mission.

YOU DO NOT HAVE TO DO ANY COLLECTION ASSIGNMENTS FOR THESE ACHIEVEMENTS! -- These missions take forever, and they will help, but they are 100% not required to get ANY of the achievements, even Liara.

See Soldier Ally for info. This achievement is for Kaiden.

See Soldier Ally. This achievement is for Wrex.

See Soldier Ally. This achievement is for Garrus.

See Soldier Ally. This achievement is for Tali.

See Soldier Ally. This achievement is for Liara.

Do a lot of side quests. If you do them all, by the first play through you'll probably get to level 40. There is a glitch that only gives you 300xp every time, but to do it: Find Liara when SHE IS TRAPPED IN THE BLUE FIELD! Kill the enemies, save, then load the save. Congrats, you got 300 points! I would only recommend that in the early levels. Also, make sure that when on the Mako to kill enemies on foot, not with the Mako, you'll get more experience that way.

You must finish the game once in order to unlock levels 51-60. In total, you will need 862,100 experience points to reach level 60. Do ALL side quests, and it is most likely that you will get this at the end of your second way through or third. Also, make sure that when on the Mako to kill enemies on foot, not with the Mako, you'll get more experience that way.

See Paragon.

On Noveria, when you are given the oppurtunity to accept the assignment from Lorik Qui'in to get the evidence to catch the Salarian (Anoleis, the administrator guy who has his own office) in his rend-based scandal, agree to it. He will open up the shut-down elevator, go down there and get the evidence. Upon exiting to go back to him you will run into Gianna (receptionist lady), so talk to her then go meet her in the hotel/bar. She will ask you to get him to agree into turning Anoleis in, agree and go talk to Lorik Qui'in. Talk to him, and do the following:

Step 1:

• (Top Right) Testify against Anoleis
• (Top Left) {Paragon} You'd be a hero
• OR
• (Bottom Left) {Renegade} Damn right I will

Step 2:

• Talk to him again
• (Left) Matriarch Benezia
• (Bottom Right) Another Question
• *Repeat step 1*

Repeat these two steps for whichever one you want until the achievement unlocks, then switch to the other for that achievement. Remember to do whichever way you DON'T want your character to be first so when you finish with the second achievement, you are high in either Paragon or Renegade, depending on your preference.

Throughout the game, you can talk to all of your squad members and learn about them. Certain squad members, if you talk with them enough, you can eventually romance. Who you can romance depends on the gender you choose to be, but the romance options are as follows. 
Male Shepard: Ashley Williams, Liara T'Soni
Female Shepard: Kaidan Alenko, Liara T'Soni

Whoever you decide to romance, make sure you talk with them after every story mission, and always choose paragon or neutral choices so as to keep the romance going. Even after they express interest, nothing will happen until the end of the game. Once you finally are able to go to Illos, your romantic interest will approach you. Continue to be supportive and eventually you will be given the option to invite them in for the night. Once the scene plays out, you will unlock the achievement.

This is a part of the story, and cannot be missed. After finishing Eden Prime, you will go back to the Citadel and you must build a case against Saren. Proceed normally until eventually you'll be told of someone who has information against him. When you find the person and meet with the Council afterwards, you will officially become a Spectre, and unlock this achievement.

There are two "impossible" situations in the game, both of which require you to have very high levels in charm or intimidate. Only focus on one, as putting points in the other is a waste. If you do not have enough, you will not have enough to resolve the situation. Both these decisions are made in the second half of the game, and contain spoilers for those have not made it that far.

Your first impossible situation is on Virmire, and it involves Wrex. When you find out that the facility Saren is using on the planet is to cure the genophage, and plan to destroy it, Wrex becomes extremely upset. You must talk with him, and try to reason with him as to why the facility must be destroyed. If you have 8 points in either of the alignment skills, you will be able to talk him down from his stance, and he will agree to keep on fighting, which unlocks the achievement.

The second impossible situation takes place during the finale of the game, on the Citadel. When you finally are able to get to the top of the tower, in the Council Chambers, you will find Saren. After talking with him, you must convince him that he's become indoctrinated, and that there's only one way to end it. Your skills need to be very high to unlock these dialogue options. If you are able to convince him to commit suicide, you will unlock the achievement.

One of the three possible places to go after meeting with the council and becoming a Spectre. Here, you must find Liara T’Soni, one of your recruitable squad members. Unlike the other planets, you are not told specifically which planet she is on, only that she is in the Artemis Tau cluster. She is on the planet Therum, in the Knossos star system. The mission starts off with you in the Mako, but after you pass through several gates, must get off and travel on foot, where you will fight a Geth Armature. After the fight, it's just a matter of freeing Liara, and rescuing her. You will then fight against a Krogan sub-boss to complete the mission.

Finish the "Bring Down the Sky" DLC missions.

You must have access to the Normandy and the Galaxy Map. Head to the Asgard System in the Exodus Cluster, then land on Asteroid X57. There are three assignments here that will help toward your completionist and ally achievements.

• Bring Down the Sky
• Avoid the Blasting Caps
• Missing Engineers

After beating the record on the 12 combat scenarios, Admiral Ahern will invite you to participate in a final scenario. Accept his offer and the scenario will begin. As soon as you start, head towards the middle of the area and retrieve the data from the large object. Then, head to the stairs where you started and activate the turret. Now, you'll need to hold out for five minutes. Make use of your teammates and the biotic powers, they'll come in very handy. Once the five minutes is up, the achievement will unlock.

See Best of the Best for more info.

There are 12 different combat scenarios you can take part in, each with different objectives. One you take first place in all of the first eight missions, you will unlock the 4 ‘advanced scenarios.’

There are four types of missions, Time Trial, Capture, Survival, and Hunt. Each individual scenario has a set requirement in order to place first in it. The requirements to gain first on each scenario are as follows.

Time Trial (Warehouse) - 01:24:67
Time Trial (Tropical) - 01:20:94
Time Trial (Volcanic) - 01:54:61

Capture (Volcanic) - 02:00:53
Capture (Tropical) - 01:58:67
Capture (Subterranean) - 01:29:47

Survival (Volcanic) - 01:31:83
Survival (Tropical) - 01:46:43
Survival (Subterranean) - 02:20:57

Hunt (Subterranean) - 35
Hunt (Volcanic) - 43
Hunt (Tropical) - 50

In time trial, you must kill all the enemies within a certain amount of time. Capture is similar to time trial, but instead of killing everyone within a specified time, you must stay near each point and capture them with a certain amount of time. Survival, you need only to survive for a certain amount of time, with it ending when you die. Hunt requires you to kill a certain amount of enemies within a certain amount of time. 

All missions (not the individual trials) found here count towards Completionist and all of the Ally achievements.