Geist

To get the rocket launcher on the multi-player map "Base", jump boost to get on the skinny platform where a man is standing. Go left from the gun turret to get there.

Wait until he shoots a missile at you. Then, get out of you body and possess the missile. Aim the missile toward him. Do this quickly, as the red ghosts will possess you and try to make you kill yourself.

To defeat this form, you must fire your weapon at the creature itself. He will be surrounded by rings. In those rings will be small gaps and you must fire at him through those. He will attack by firing the rings at you. Get either up or down in order to dodge the ones he fires. Once you get him enough, there will be four gems. Shoot the bottom left, bottom right, and the top gems first. Once they stop glowing blue, attack the one in the middle. He will then start losing life. Once the gem is destroyed he will revert back into his demonic form. Repeat this until the creature's life has been completely depleted.

Shoot the pieces of stone that are glowing. After you have shot all of the pieces, keep grenading him with the secondary fire of your gun that you have.

This trick requires a controller with an auto-fire feature. Set the auto-fire to a button. Every time you get caught by them, just hold a button. This helps when fighting the Boss that takes over Rouqe's body.

During Level 8 Rourke, after you’ve taken possession of Rourke, head to the Command Station. (After you’ve gotten off the elevator, head down the hall. The Command Center is the door in front of you, and a guard should be on your left guarding another hallway; he escorts you to your new armor.) As you enter the Command Center, look at nearly any computer with a blue screen (there are 7 of them, 3 are actually working). Examine the screen and it will say 'The operating system has crashed on this computer.'

Look in one of the lockers in the girl's shower room to find a Gamecube.

To defeat the level 4, Medical, Boss, when the fight starts run around the room, dodging its projectiles and shooting it in the head. It will not do much damage, but it helps. At certain times, it will stop and shoot a huge wave of projectiles at you. Try your best to get in between two projectiles to avoid it. Tip: When it is about to do it, it will wiggle its head around as if it is in pain. Watch out for this sign. Eventually it will kneel down and put a blue shield around its head and start healing itself. Quickly run over and throw a grenade into the hole in its shield to do lots of damage. If you do not, it will heal itself back to full health. Immediately after you do this, quickly back away as it will try to attack you with its claws. If you are low on health, you should get one of the three first aid kits at this point. This fight is going to get a lot tougher. Immediately after you do this, it will change its tactics. It will start shooting more projectiles more often and instead of shooting a wave of projectiles, it will emit a huge orange electrical beam from its mouth. Stay far away from it, as it will sap your health quickly if it hits you. The last thing it does is get out of the middle of the arena and roll around the room. It either rolls around near the center of the room or the far side of the room. Indicate which part its rolling around to avoid it. Keep repeating the strategy to bring down its health. When it is almost dead it will roll around and stop to reveal an opening. Throw a grenade into the opening to do damage. Eventually you will defeat it.

For every two Host Collectibles that you find in the single player game, you will unlock an extra feature for the multiplayer mode in the order shown below:

Office Multiplayer Level
Storage Multiplayer Level
Raimi and Rabbit Characters
Medical Multiplayer Level
Towel Girl and Bat Characters
Helipad Multiplayer Level
Chef and Rat Characters
Raid Multiplayer Level
Dead Garden Multiplayer Level
Summit Multiplayer Level
Volks and Roach Characters
Complex Multiplayer Level
Heaven Multiplayer Level
Boss Multiplayer Level
Anna and Imp Characters
Catacombs Level

To get these, you have to find the Host Collectables in each level. Each unlockable needs to Host Collectables, and there are four per level.