Veteran difficulty is available from the start and it would be a good idea to start there since the difficulties do stack. Starting here will give you a decent idea of what to expect on your Professional playthrough.

If you have a friend who has already unlocked Professional, you can skip this step and begin there, but you won't have access to any guns with unlimited ammo, which is extremely helpful on your Pro playthrough. You can ride on your friend's coattails though.

After beating the game on Veteran, Professional difficulty will unlock. You will also have the ability to unlock unlimited ammo in any gun you have completely upgraded. You must purchase this ability with your "exchange points" which are given based on your grade in each chapter, and it only affects the specific guns you've used your exchange points on. The feature must also be turned on in the Bonus Features menu.

If you want to take that even further and unlock unlimited rocket launcher ammo, you can replay the game and beat it in under five hours (obviously easiest on Amateur difficulty).

You must have all four types on eggs in your inventory at the same time. You can get this quickly if you have a friend with all the eggs, just have him give them to you. Otherwise, here are the first instance of their locations. Except the gold egg, they can all be found in multiple spots.

White - Chapter 1-2. Just before you head into the building with the lady in the black dress head up to the roof of the building which has the shotgun. Should be a chicken up there, if not he's already roaming about. Drops as white egg on Normal difficulty.

Brown - Chapter 1-2. Same as "white" but play on Amateur.

Gold - Chapter 3-1. Middle island on the map, the "chicken farm." Arrive and wait a moment, chicken should give a little cock a doodle do and drop it.

Rotten - This drops randomly later in the game from Chapter 4-1 onwards off defeating enemies.

There are two ways to earn weapons in the game. First, you can find silver cases throughout the game and pick them up, or you will unlock extra weapons by upgrading exisiting weapons. If you are playing co-op and your partner picks up a weapon, it will then be available for you to buy, or you can just go back into the level and pick it up for yourself later.

You must have one each of the following in your inventory at the same time:

Hand Gun
M92F - Found in Chapter 1-1
H&K P8
SIG P226
M93R - Fully Upgrade M92F

Shotgun
Ithaca M37 - Found in Chapter 1-2
M3 - Found in Chapter 3-3
Jail Breaker - Found in Chapter 6-1
Hydra - Fully Upgrade Ithaca M37

Machine Gun
ZV61
AK-74 - Found in Chapter 5-1
H&K MP5 - Found in Chapter 2-1
SIG 556 - Found in Chapter 5-2

Rifle
S75 - Found in Chapter 2-1
Dragunov SVD
H&K PSG-1 - Found in Chapter 5-3

Magnum
S&W M29 - Found in Chapter 3-1
L.Hawk
S&W M500 - Fully Upgrade S&W M29

Most of the action figures will be unlocked by completing the game and finding all 30 BSAA Emblems. However, two require you to S-rank every level. This isn't very difficult once you have unlocked unlimited ammo on a magnum and go back to play the game on Amateur. You are ranked by accuracy, enemies kill, deaths taken and time to complete. Just blast through every enemy in solo mode (don't give Sheva a weapon) and you shouldn't have any problem getting an S-rank. Once you unlock the final two, the achievement will pop after purchasing them.

This will need to be performed on a Big Man Majini and is best done co-op since the AI won't really help you. Shoot him once with a weak shotgun (so you don't damage him too much) to staggar him, then have one person behin the chain by hitting X; then the other person will do the same; finally, the first person will have a special attack to finish them off.

This does not unlock for both players, only the person who hit twice and used their special attack. Make sure to return the favor to your friend!

Again at the end of Chapter 5-3 just after "Bad Blood." Wesker will depart and leave you alone with Jill. Move closely to her but be careful, too close and you'll get a boot to the face. The game will prompt you with the button to try and reason with her (if you are Chris); it may take a few of these prompts but Jill will then become confused. Sheva will grab Jill and give you the opertunity to try and grab the chest piece or shoot it, shoot it for now until it begins sparking. Now reason with Jill and when Sheva grabs her press to grab the chest piece then mash to try and rip it out. It will take a few attempts of this to finish the battle.

This will not unlock if you rip out the chest piece when Jill is on the floor.

This achievement MUST be done in solo with the AI.

If you want this very quickly, load Sheva and yourself up with only healing items (buy spray cans if needed), then load Chapter 1-2 and shoot the transformer to the ground. Stand next to it and knife it to hurt yourself so she runs over and heals you. After she heals you, press to say "Thanks!" and her trust will go up. Repeat this until you get the achievement.