X-Men

100 lives:
Press B(5) at the title screen. The sound of a bark will confirm correct code entry.

Final stage:
Complete all stages except for the Practice stage. Then, highlight "Practice" at the stage selection screen and hold Select + Up + B and press Start.
Information in this section was contributed by rick91.

Regenerated X-Men:
If you happen to lose any of your X-Men during a battle, they all can be restored. This works well when all you have left of a team is Colossus. Go into practice mode and start a game. Then, whenever you want, you can leave and get the wounded X-Men back by pressing A + B + Start. You can then go back and play any levels that you have not completed with a full load of X-Men.
Information in this section was contributed by LilJay11288.