When you enter a room with a creature in it, run around until you're behind it, then attack. The game will say "advantage" at the top of the screen, and you will come into battle fully charged. Now you can hit the creature in the back, which does more damage.
Beat the game and after each saving point save, and restart the system. Each time you do this, change the resolution. You must begin with the resolution set on HIGH RESOLUTION, LETTTERBOX, then cycle up from there. This will add a cheat menu at the title screen.
After beating the game on the ultimate difficulty, press the following at the title screen: L, R, L, R, START. This allows you to play as President Weller.
After beating the game on the ultimate difficulty, press the following at the title screen: L, R, L, R, Z. This allows you to play as the alien who wears a red dress.