All right. In the options menu adjust the difficulty to deadliest. Simply play through the arcade mode as the Ninja, Viking, and Knight.
I found the Ninja's combo X,X,Y,Y was very effective. The Viking can one shot kill enemies with his projectile. Learning the timing for this move is extremely valuable.
The Knight gave me lots of trouble. He's slow and gets knocked off balance after a hit. What I had to do was play the arcade mode with the knight on normal to unlock the mace. Equip the mace and use A,A,Y to level the playing field.
If you ever face an opponent that gives you a bunch of trouble, launch your projectiles at him then run around the map until runs out. Cowardly, but effective!