Boss code:
Choose any character then at the versus screen, just before the fight starts HOLD right on the D-pad and quickly press SELECT, START B,A.

Eyedol's Thrasher Finisher:
When the opponent is woozy or in "danger", jump in front of him/her/it.

Fight Eyedol:
You can only fight Eyedol if you play the game on easy, medium, hard, or very hard, not very easy though.

Thanks to Revolution readers Alex and Michael DePue!