Defeating Night Crawler
Knock Night Crawler onto the spikes that appear from the floor. Keep punching him, and the spikes will help kill him quickly, often without hitting you back.
Cyclops
Choose Cyclops when selecting a character. He has no punch combo, so you can continuously punch one enemy till he is KOd without having to stop, unlike the other X-men who can only punch three or four times. Note: This will not work on most Bosses. mini-Bosses and Sentinels.
Special move
Press A + B to do a special move. Note: This takes away a little bit of life.
Play As Bosses
Complete the single-player mode. When you hook up the Game Boy Advances and select a mode, you can choose from Bosses (but not mini-Bosses).