Cheat:

 This code will let you use Magneto and Juggernaut. To do it,
 simply go to the option menu by pressing [F10] and then go into
 the configuration menu. Once there, type in the word SPAM. After
 typing in the code, go into system and you will see a new option
 called BOSS SELECT. Enable this option and then save the new
 settings. When saved, start a normal game and select a fighter
 and block mode. Now just before the battle loads hold all three
 punches down (if you want to use Juggernaut) OR hold all 3 kicks
 down (if you want to use Magneto.) Continue to hold down the 3
 punches or 3 kicks until the battle finishes loading. From then,
 you should be able to use your new boss character.