Start game as usual. During gameplay press [~] to go to 
console, then enter the following cheats:

Codes - Result 
elvis lives - Become Invulnerable
no i am winner - Win Level
oppenheimer - Extra Powerful Fireballs
xmas tree - Get Lots Of Bonuses
tabasco - Shoot fireballs
system visor - Show Game Info (FPS etc)
i love carnage - More Monsters ?