Brute Force Cheats

Go to cheat menu and enter...
BRUTAL
to get a harder difficulty put complete game on this hardness, and SAVE IT then you can play it with 5 weapons
on game hold down on Y to go though weapons

This is good if ur running low on medkits in a hard level like
killing shadoon or seers in the Bayou. This Costs you cash and Ops
But you'll be able to get your ops back depending on how many
controllers you have. First hook up all the controls you have and
go to the level. when your running low on those valuable medkits
kill all your ops except one. press start and pick your character
on the second, third, or Fourth controller to get back an op. when
you get back an op it'll have an extra medkit with him/her. keep
repeating this process until you have the amount of medkits you
need. If you want to get all your ops back you must have four
controllers press start on all of them pick that character and
then press start and quit on the to get your ops back to AI bots.
this cheat costs cash but it's worth it on hard
levels    

Make sure to tell the other characters to support you when attacking a frag.
When in combat, tell your teammates to fire at will, move around a lot, and throw grenades when there are a lot of aliens grouped together.
Always pair one guy with one girl when playing regularly.
To clear out a lot of enemies, have Tex and Brutus go in and do as much damage as possible. Then, retreat back to Flint and Hawk. Also have Flint give you sniper support and have Hawk cover Flint. It also helps to send Hawk in invisible to scout and kill as much as possible.
When going into any hallway or cave, keep Flint back, but able to snipe clearly into the hall or cave. Put Brutus on one side and Tex on the other outside of the hallway. Have Hawk explore, and if needed lead the enemies out. They will be blown away from front, left and right and from Hawk.

If you don't have anyone you can play with and you are forced to do this yourself, you can have a relatively easier time (and save ammo) by simply using Hawk (the Assassin) to kill one or two enemies while cloaked, then retreating to regenerate energy, then doing this over and over until you need the really heavy firepower of your AI buddies.

Regenerate special energy quickly by crouching (push in the left joystick, or as the PS2 dorks call it, 'Press and hold L3').
Submitted by - Truzen
Unlock Characters
Unlock Confed Marine - Find the DNA sequence in Mission 1 or Mission 6
Unlock Feral Colonist - Find the DNA sequence in Mission 2
Unlock Feral Outcast - Find the DNA sequence in Mission 3
Unlock Seer Follower - Find the DNA sequence in Mission 4
Unlock Militia - Find the DNA sequence in Mission 5 or Mission 11
Unlock Outcast Shaman - Find the DNA sequence in Mission 7
Unlock Seer Priest - Find the DNA sequence in Mission 8
Unlock Feral Shaman - Find the DNA sequence in Mission 9
Unlock Gunthar Ghent - Find the DNA sequence in Mission 10
Unlock Shadoon - Find the DNA sequence in Mission 12
Unlock Fire Hound - Find the DNA sequence in Mission 13
Unlock McTavish - Find the DNA sequence in Mission 14
Unlock Shrike Soldier - Find the DNA sequence in Mission 15
Unlock Shrike Heavy - Find the DNA sequence in Mission 16
Unlock Shrike Hound - Find the DNA sequence in Mission 17
Unlock Hunter Lord - Find the DNA sequence in Mission 18

Put in one of the following names for your profile name:

BRUTAL - Brutal Difficulty
DBLDAY - Die fast

DEADAIM - Aim better

ERINROBERTS - Better Defense

HVYMTL - Get Cartoon Mode

MATTSOELL - Tougher Characters

RAPIDFIRE - Rapid Fire Weapons

SPRAGNT - Stupid AI