Area 51 (Xbox)

Defeating the Thetas
The best way to defeat a Theta is to first mutate. Next, fire parasites; or if you want to take a chance, fire contagion to lay some heavy damage on him. Use contagion for heavy damage, but parasites also work well, especially because they heal you. When your mutagen runs low, run up and melee him and the enemies he releases on you. Keep doing this and he will eventually go down. This will take some time to accomplish and may be difficult at first.

Unlock the Alien Multi-player Avatar
Beat the Campaign mode on the Hard difficulty setting to unlock an alien Gray as a selectable multi-player avatar.

Hard difficulty
Complete the game on normal difficulty to unlock the hard difficulty.

Sharks with machine gun heads
In the level "The Grays" proceed into the 4th room in the level, where you can see the monorail you entered the level from out the window. Push the computer console there and it will say "Lifter activated". Make your way back outside to the monorail, you can now stand on it. Run all the way to the back of it and you will be warped to a weird room with sharks that have machine guns attached to their heads

Unlock Final Secret
Grab all the other secrets to unlock the final secret.

Unlocking Secret Tapes
Scanning all five databank items in each stage yields one secret. Completing the game yields one secret.

Upgrading The Alien Grenade
In the stage Life or Death, there is a small key on a dead soldier in one of the lifts where Cole starts the stage. Grab it to open the small lockers in the stage. One of the small lockers in the first power conduit pit has a databank item that upgrades the alien grenade.