(Some of these only work in 2 player game)

Temporary invincibility:
away, away, away, away, attack    

Temporarily change opponents controls:
forward, forward, forward, forward, attack

Play military vs. military:
At main screen, press left, then right, then left, then right 
repeatedly until the screen flashes.

To fight the supervisor:
At main screen, press left, left, right, right, then repeat until
screen flashes.
Then you can play either against the supervisor as player 2, or go to
training and play with the computer as supervisor.

Supervisor's moves: 
down-toward-up: praying mantis
  down-away-up: melt and heal