Use shift to pause the game and then click on the red skull to enter cheats.
Code:
valkyrie - Kill Combo 100 (Once per round)
pantheon - GodHand equipped (Once per round)
rambo - Minigun equipped (Once per round)
adrenaline - Second wind ??
super saiyan - +3000 exp points (Once per game)
i am legend - Kill Count starts at 3000 but no exp for the round.
doom - MFG equipped (Once per round)
ultima - ???
ijohnf - John's head (Restart round)
team meat - Meat Boy head (Restart round)
zombified - Walker head (Restart round)
wtf - all seeing eye (Restart round)
Most can only be used once per round or even per game so not much help, but the game is AS2 so it should be easy to hack!