These achievements stack, so do the game on Legendary first to save yourself doing the game more than once. It can and will be frustrating at times, but perseverence is key. If you are stuck at any point, use this guide for help. It is an amazing walkthrough with videos and it gives tips on parts where you can skip entire battles, which is extremely nice.

General Tips:

• The plasma pistol/battle rifle (aka noob combo) is the best thing in the game. Hit them with a plasma blast, then switch to the BR and headshot, this will kill most enemies in one go.
• Your shields recharge, make use of it. Get out of battle if you've been shot, more will follow soon after.
• Use your NPCs. Let them go into battle first to draw fire, then you can attack while the enemies are focused on them. Also, giving them a good weapon will help, since they won't run out of ammo. If you have a rocket and there are a lot of vehicles attacking you, giving it to the NPC and letting them fire rocket after rocket will help.
• Grenades are clutch. Make sure you make every grenade count, since you can't carry many, but they will do a great deal of damage. If you have stickies, make sure you get them to stick or the enemies will most likely get out of the way.

If the Banshee gets stuck, shoot off its wings (don't melee, shoot). Take it down the side tunnel to the left. As soon as it's says "Loading.... Done", board the Banshee (if you don't it will dissappear). Fly out and up, up high in the sky there are two bridges between two skyscrapers. Fly to the middle of the one closest to you and you should see a Plasma Rifle floating in the air a little bit above the ground. This is the Scarab Gun.

Use this video for help:

There are two very easy ways to get this. On "Delta Halo" you will get a checkpoint, then come to a jackal with a sniper standing above a large clearing that you can assassinate. Do that, reload the checkpoint and do it six more times. Even though you reload the checkpoint, it counts them.

The other spot is when you come to a large open room (I forget the level) and there are flying enemies attacking some Covenant across the way. If you have a sniper, simply snipe seven of the flying things for the achievement. Leave the Covenant alive so the flying dudes stay attacking them and don't look your way.

Another spot is on "High Charity" when the drones are fighting the flood. Kill the drones during this section. 

Note: "From behind" does not mean you need to melee them, nor does it mean you have to hit them in the back. You can snipe seven enemies in the face and still get this.

This achievement can be tricky legit, since you have to find someone who is on a Killing Streak with only the sword, then make sure you kill him while he has the sword out. If he switches to another weapon, he is no longer a "sword carrier" and it won't count.

Maps: Lockout, Ivory Tower, Midship, Warlock

Note: You can do most of the multiplayer achievements with yourself and one friend by starting a game and just letting each other do whatever is needed.

Grab a rocket launcher during any gametype and hold the trigger to lock on and blow it up. Extremely simple.

Maps: Ascension, Coagulation, Waterworks, Headlong, Containment

Here are the types of medals you will need for this:

• Sniper (snipe someone)
• Sticky (stick some one with a plasma grenade)
• Assassination (hit someone in the back)
• Beatdown (kill someone with a melee)
• Double Kill (kill two players within 4 seconds)
• Killing Spree (kill five players without dying)
• Hijack (board a vehicle)
• Road Kill (run someone over)

Note: You will need at least three people to boost this achievement because of the double kill. Also, getting a double and a triple kill only counts as one medal, same with killing spree and killing frenzy. Those just take your highest medal and give it to you, not both.