Unlock Multiplayer Characters
Collect the DNA containers throughout the levels to get more characters. Note: DNA Canisters are always in the final chapter of a mission.

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 to those little blue huts: The commander guy says something like "I got a good signal on this place" then powerups come up. Then whenever you go in a little blue hut, powerups will be bountiful - too easy!

Effective weapons
Good weapons against the shadoon are shotgun, or any projectile (teleporters die faster with energy). Bad tools against them are bio weapons.

Good weapons against the lizard things are bio weapons, minigun, shotgun, sniper rifles. Bad weapons are pistols and smaller energy weapons.

Good weapons against the mutants are sniper rifle, energy weapons, minigun, powerful pistol. Bad tools are bio weapons.

Good weapons against terrorists and marines are basically all heavy weapons, and a small number of handguns.

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

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

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

Hint: 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).

Hint: Brutus:
For all enemies except the last Boss, use Brutus to charge the enemy. He only looses 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.

Hint: 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 some ash. She will not have any blood because she is a robot or a cyborg, 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 snipe 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.

Hint: 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.

Hint: 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.

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. Booyah!

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