When you are in the room at the start pick up the rocket luncher before you enter the teleport.
When you get out go up the top & hit the switch that makes the lift come up.
Go down @ get on the lift. When it is going up fire a rocket into the big demons brain it will make a loud noise if you get the rocket in.
Do it about 3 times and the demon will die. Another way to do it is to type in idclip and walk into the demon you will be in a small room with a head on a stick, shoot the head and the level ends.