When you arrive at the moon base, you'll be faced with two locked doors: one to the grinder room and one marked with a pentagram. Also nearby are three consoles containing science logs. Read all three logs, and the grinder room door will open.

Inside the grinder room, you'll find another console. Read the log, and the pentagram door will be unlocked. Leave the grinder room and wait for the blood diversion sequence to finish. Go through the door and into the teleporter.

When you arrive at the crater, you must jump down the pillars to the platform where Satan is recuperating. Only Bob can make these jumps, so leave the host behind.

When the battle begins, run to the other side of the platform and possess a worker. Satan's minions will possess cops and start firing at you, but you have a more important task at hand.

Run around the platform and activate all the consoles on the valves. Satan will occasionally unleash a ring of fire, but as long as you are in a host you should be able to jump over it.

When all the consoles are activated, Satan's shield will come down. He'll come after you. To hurt him, you must depossess a creature in his vicinity, which will send a bolt of spiritual force at him. There should be plenty of hosts around, so you should have no trouble besting him. Just don't let him get too close.

Once you defeat Satan, you've won the game. Congratulations!