Level 1: Destroy tank at end
You will face an alien tank at the end of the first level. The easiest way of destroying it is to just charge straight at it. Fire like crazy while targeting it. If you run out of ammunition, take out your blade and kill a nearby Borg. Once it explodes, kill the remaining Borgs to win.
Level 1: Collapsed gantry
When The gantry collapses and you turn on the power, the electric cable nearby will electrify the gantry. If you try to cross, you will get hurt, and it is your only way back. Instead, face the burning trucks and press R2 to switch to first person view. Scroll left, look at the top of the building, and shoot out the electrical box. This will cut off the electricity on the gantry.
Defeating Alien Troopers
Hold R1 and run around them, shooting them with the laser. If they are in a group, they usually will not be paying attention. Just stand back and shoot them in the head.
Defeating Alien Spiders
Stand back, fire one secondary rocket launcher attack, a grenade (secondary machine gun), and finish it off with a laser in the head. Be careful of the small ones. Stand far away and blast them with the machine gun.
Defeating the Boss in the Alien Base
After Dr. Carter gets captured by the aliens, you must make your way through mazes and buildings to find her. When you do, her father, who has turned into an alien himself, sends a war drone after you, The best way to defeat it is to destroy the laser arm on the left, followed by the one on the right. Every now and then, pick up the items lying around. Once they are gone, destroy the Drone. If the laser arm starts to fire at you, or the drone brings out its machine guns, just press Triangle to use your shields.
Unlimited Ammo
After pausing the game HOLD L2 and press Down, Left, Right, X, Square, Circle.
Invincibility
After pausing the game HOLD L2 and press Up, Left, Down, Right, Triangle, Square, X, Circle.
Get Stealth Mode
After pausing the game HOLD L2 and press X, X, Square, Square, triangle, Triangle, Circle, Circle, X, X.