After beating the game under the Ultimate Difficulty setting, enter the following
codes at the title screen:
Extra Mode L, R, L, R, A
Play as Alien L, R, L, R, Z
Play as President Weller L, R, L, R, START
When you enter a room with a creature in it, run around until you can get behind
it and then attack. If you did it right, the game will say "Advantage" on the
top of the screen and you will enter the battle fully charged. You will also be
able to hit the creature in the back (it does more damage when you hit a creature
in the back).
Thanks to Revolution readers El Cujo and Rob!