Gun power-up
You will lose a gun power-up as soon as you shoot one of the other humans. This is because when you lose a life, you lose your gun; and when you shoot a human, you lose a life.
Baby Aliens
When you enter the building from the helicopter, shoot the exit sign when you see a purple alien, then shoot the first three lights. This gives you the opportunity to shoot baby aliens before they form.
Chow Palace
When you get to the point where you fight the two grenade throwing aliens, kill them and the game should give you enough time to destroy everything around them. There are yellow drums and ammo crates at this location. There is a single alien and about fifty power-ups inside Chow Palace.
Shoot all of the back windows in the warehouse after destroying the helicopter to enter the bonus stage.
Tank Top
Shoot the first fifteen windows on the right side of the hanger in wave two, just after you enter. You will need two players to do this, and you must use several views of these windows to hit them all. This entrance leads to a room called Tank Top.
X Marks The Spot
Shoot the first ten hanger windows while outside on the tarmac. The first two windows must be shot quickly, in transition, immediately after the game starts. This leads to a room called X Mark The Spot.
Killing the launching opponents
Whenever you encounter the types of opponents that launch things at you, shoot at their head. Whatever they are shooting at you will also be hit, as if they were targeted. Just keep aiming at their head and they will eventually be killed. Note: This will only work on one person at a time.
Shotgun
When you are in the warp ahead level, get up to the part where it says "Self Destruct". You will turn, a blue blinking light will appear. Shoot it and you will get a shotgun.
Whenever you receive a shotgun power-up you can simply aim in the area around the target to hit them. You do not have to aim exactly on target.
Secret rooms
Enable the "Secret rooms" option. Do any one of the following to enter a room called Chow Palace, which is filled with power-ups.
Inside the bunker, shoot all the blue lights on the left wall.
Shoot out all the back windows inside the bunker.
When riding inside the forklift, shoot all the yellow barrels.
When riding inside the jeep, shoot all the yellow boxes.
When you are in the building shoot the name plate off the second door as it is opening.
When you are being bombarded by the two aliens on top of the hill, shoot everything yellow.