To unlock characters for Multiplayer, collect DNA containers stashed throughout the levels.
Single Player Breeze
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.
Bust 100%
In order to achieve 100 percent completion on a mission, you have to play on the Brutal difficulty setting. Complete all primary and secondary objectives, kill every single enemy and find the hidden DNA sequence. Also, you can't allow any of your teammates to die.
Earning Medals
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.
Regenerating
Here's a little cheat to regenerate your teammates. 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.
Regeneration Glitch (Special Energy)
Regenerate special energy quickly by crouching.
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
How to beat the final boss
The final boss at first may be difficult but he really is easy. Get near the shield that he teleports him self to. If you look around near there you will see those generator things you had to shoot earlier in the mission. Shoot them all and then go to were the console is in the middle of the area your in. When he teleports back to the shield was, activate the console and a meteor will crash into him killing him.
Easy powerups at lava levels
You can get easy ammo, grenades, and medpaks in those lava levels(the first lava level you get Hawk)by going those little blue huts the caammander guy says something like "i got a good signal on this place" then powerups come up. Then whenever you go in a little blue huts powerups come.
Squad death match teams
Have you wondered how to get squads for squad death match? Well what you have to do is get all the members of the squad. Like for the platoon you have to get Guther Ghent, Ty McTavish, Confed marine, and militia.
Easy Attacking
Use your other characters to support you at the flanks, this will make it easy for you to attack.
Stupid AI
Enter SPRAGNT as your profile name.
Rapid Fire Weapons
Enter RAPIDFIRE as your profile name.
Tougher Characters
Enter MATTSOELL as your profile name.
Get Cartoon Mode
Enter HVYMTL as your profile name.
Better Defense
Enter ERINROBERTS as your profile name.
Aim better
Enter DEADAIM as your profile name.
Die fast
Enter DBLDAY as your profile name.
Brutal Difficulty
Enter BRUTAL as your profile name.
Further Grenade Distance
Get any type of grenade look straight up slightly look towards where you want to chuck it and hold the left trigger down and let it fly. It should go further but it isn't that accurate.