Stronger characters
Create a new profile and name it "MATTSOELL". When playing under this filem Tex and Brutus can take more than three direct shots of the rocket launcher without being near death. Note: "MATTSOELL" is the project director for Bungie who made Halo. Also, you may need to see the ending once before this code has any effect.

Rapid fire weapons
Create a new profile and name it "RAPIDFIRE". All weapons, even the shotgun, will fire at a higher rate of speed. Note: Ammunition is eaten up much faster. Note: You may need to see the ending once before this code has any effect.

Dumber enemies
Create a new profile and name it "SPRAGNT". Even under the Brutal difficulty setting, enemies will miss you more often, act stupid, and your squad will be more intelligent and shoot and kill enemies with no instructions. Note: You may need to see the ending once before this code has any effect.

Better aim
Create a new profile and name it "DEADAIM". You will notice your teammates aim and hit things much more often. Flint's special meter will decrease slower, and she will drop enemies quickly. Note: You may need to see the ending once before this code has any effect.

Higher defense
Create a new profile and name it "ERINROBERTS". This somewhat increases your squad's defense in damage taken. They will not get hurt as often, but also seem to dodge projectiles. Note: You may need to see the ending once before this code has any effect.

Quicker death
Create a new profile and name it "DBLDAY". Everyone will be easier to kill; your squad may also die much faster. Note: You may need to see the ending once before this code has any effect.

Tougher game
Create a new profile and name it "BRUTAL". This code completely beefs up the enemy AI and seems to change their behaviors. Even under the Standard difficulty setting they will ambush you, find a way to hunt you, and come flailing out in order to kill you. They will take the same amount of hits, but be much harder to put down, thus making the entire game harder. They drag themselves away when wounded, similarly to the original previews of the game or when you fight the Halo aliens under the Legendary difficulty setting. Note: You may need to see the ending once before this code has any effect.

Bots in multi-player
Start a game of Squad Deathmatch. Plug in controller two and set the A.I to "Fire At Will". Go around and pick them up some good weapons and medkits, then disconnect the controller. Your squad will now be facing three heavy thinking bots in multiplayer mode.

Keep items
If you lose your life in single player mode, join the game using controllers two, three, and four to get them back.

Maximum MK ASLT ammunition
When you find a duplicate MK ASLT, keep picking it up. It will raise your maximum ammunition to 600.

Restore special energy
Click the Left Analog-stick to crouch and restore energy faster.

Brutus
For all enemies except the last Boss, use Brutus to charge the enemy. He only losses his Vengar when the enemy is dead. You can charge them and damage them multiple times, which is very effective. Also, use Brutus' specialty to find snipers before they find you. Just flick it on and off.

Flint
If one of your characters dies, notice that they have red blood surrounding them. However, whenever Flint dies, there will not be any blood, only a black spot from the gun debris and ash. She will not have any blood because she is a robot or a sentry, as she mentioned earlier in the game. A good way to see this is to go directly to squad fight. Do a Brute Force against another Brute Force team. Have one of the team's Flint sniper all of its teammate's characters except for one. To save time, aim for the head. Then, have the other character who you left standing shoot Flint and kill her. If you keep shooting her body while its on the ground, it will not bleed.

Hawk
If you do not have anyone you can play with and you are forced to do this yourself, you can have a relatively easier time (and save ammunition) by simply using Hawk (the Assassin) to kill one or two enemies while cloaked. Then, retreat to regenerate energy. Repeat this until you need the really heavy firepower of your AI companions. Also while playing as Hawk, select your PowerBlade "PwrBld" in your inventory. Use her specialty (cloaking) and stab as many enemies as you can. One stab in an enemy kills them, and since Hawk runs very fast, you can take out many enemies without taking any damage.

Tex
When you select a character, notice that some of them have larger health and special meters. When you select a difficult level, unless you charge up Hawk's stealth meter, you should choose Tex. He has the largest health/special meters.

Defeating the final Boss
Use Hawk to go to the farthest yellow map marker and take everything out (enemies, turrets, and force field). Activate the console. After the door opens go to the room with the turret and destroy it. Activate the console then go out and through the door that opened. This will lead you to the second yellow dot, but stay out of the Boss' line of sight. Take Brutus up to where the farthest yellow dot is located. Use Vengar or you will probably get killed. Use Tex as desired, as he is not essential for this battle. Finally, use Flint to snipe the Boss, who should be distracted by Brutus who just ran to the opposite end of the map. Shoot him in the back until he warps to the place where he powers up (where the Blue Power Orb was located before it was destroyed). Take Hawk to the command console and activate it (the closer yellow map dot).

Explore asteroid in Hostile Reception
When leaving the last tunnel and returning to the outside, notice the turret located on the tunnel you just exited. Face the tunnel and walk to the right side of it. Follow the rocks to where the rock wall meets up with the tunnel's outer surface. You will fall through a small gap in the asteroid. It may appear you are stuck; just "wiggle" slightly. From here, you can walk around the asteroid and revisit some tunnels.

Bonus multi-player characters
Get the hidden DNA containers to unlock various characters in multi-player mode:

Confed Marine: Mission 1 or Mission 6
Feral Colonist: Mission 2
Feral Outcast: Mission 3
Feral Shaman: Mission 9
Fire Hound: Mission 13
Gunthar Ghent: Mission 10
Hunter Lord: Mission 18
McTavish: Mission 14
Militia: Mission 5 or Mission 11
Outcast Shaman: Mission 7
Seer Follower: Mission 4
Seer Priest: Mission 8
Shadoon: Mission 12
Shrike Heavy: Mission 16
Shrike Hound: Mission 17
Shrike Soldier: Mission 15
General tips
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.