For each of the cheats listed below, enter it as your profile name when beginning in order to activate the cheat:

BRUTAL: Harder Difficulty
DBLDAY: Quick Death
DEADAIM: Better Aim
ERINROBERTS: Better Defense
HVYMTL: Cartoon Mode
MATTSOELL: Tougher Characters
RAPIDFIRE: Rapid Fire Weapons
SPRAGNT: Stupid AI

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

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.

To regenerate a character's special power faster, simply crouch down.

If you're on your last guy, insert another three controllers into your Xbox and press start. As new players join, your teammates will be brought back to life. Now just quit out of the three games you just opened and you'll go back to single player mode, but your teammates will remain.

At the mission select screen, you'll see four spots for medals. The first three spots are filled when you beat the mission on Standard, Hard and Brutal. The fourth is filled when you find the hidden DNA sequence. If you've completed all objectives and killed all enemies, the medal will be gold. If not, it will be silver.