Gold Cheat:
To get gold you must beat the game on hard. Winning gold will give you infinte ammo, all weapons and all the rest of the multiplayer codes you recieve when you win silver and bronze.

Silver Cheat:
To enable Silver Cheat, finish the game in MEDIUM difficulty. This will open 2 more bars in multiplayer mode : Game Speed and Blast Force. Plus it will unlock the multiplayer cheats you get for winning bronze.

Bronze Cheats:
Beat the game on the easy difficulty. This will give you two options for multiplayer mode: Weapons Stay and One hit kill.

Invincibility:
Pause game and press L2, L2, R1, R2, R1, L2.

Kill Final Boss easily:
First when you enter the boss room strafe one step to the right so you see the tank commando and shoot him down with your rocket launcher when he's dead run forward to the flying enemy and run from right to left and shoot at him with your hyperblaster and when you dont have any more ammo take the minigun and do the same thing again. Then you shoot the other tank commander with your rocket launcher and when do the same thing to the flying enemy

Hidden multiplayer areas:
In the Hydophobia level, go into that hallway thats all red. Stop in the center and you'll notice a darkened area on the wall. Shoot it and it will reveal a BFG10K. There's another secret. In the pool of water by the super-shotgun, jump in and shoot the center of the wall by the shotgun to reveal a quad damage.

In Toxic Vats level,go into the narrow hallway with a ladder on each side. Take the ladder that brings you to the platform where you can get on the moving crane thingy. Take the hallway to the part where you're standing on the glass platform with the hyperblaster on it. look down through the glass and you'll be able to see a railgun submerged in a small section of the toxic waste. Jump in and swim down and get the railgun.

Infinite ammo:
Take the chaingun, which is weapon number 5 in the list, and then press the fire button. Keep holding it down, and then change your weapon to anything else. Now you should have unlimited ammo on your chaingun AND machine gun if you did it correctly. This may take a few tries.

Hints:
If you want to save a game whenever you want, just walk back at the beginning of the level, there you can save.

If you need more bullets for machinegun or vulcan (you know the big rotating one) take the vulcan. Then shoot and hold it, during that choose another weapon. Now you should have unlimited ammo for those two guns. The same you can do with the hyperblaster in multiplayer level toxic vats (you must finish the game on medium first)

Walk on the place where you can find the hyperblaster, then shoot till you have 37 bullets left. Then switch to machine gun, do the same trick as above, now you should have unlimited ammo for the best weapon in the whole game.

If you really want to beat your friends on multiplayer, first enable one hit kill. After that begin the game, run like mad to get the hyperblaster to defend yourself for a couple of minutes. Then go after the BZG when you have it, simply aim at the ground and jump (set gravity on zero, or else you are toast), try to control and then you have to shoot but you must blast of when you are in the top of your jump. If you shoot then you are able to fly for about one minute. You can navigate yourself while flying to look up and down, walk forward an backwards. If you are very good at it, you can even make a salto. Imagine yourself what happens when you make a looping 360 degrees and shoot at the same time with a hyperblaster.

Before you walk into a corridor or a small section always check the walls. There are over a hundred secret rooms in the 1st three levels.

If you walk on the 6th level (maybe 7th) then you will see a big hole under you lava in it. You can walk over it by enable a button which will make a bridge, first you must do that. Then do something unexpected: Throw yourself into the lava! You can swim in it. Be carefull! You only have 15 seconds, before you burn out. You will find an BZG weapon, the green beam cannon blaster from multiplayer! This weapon is very handy against the tanks and ecpecially the tank commanders.