Alternate costumes
At the main menu, quickly press Up, Down, Up, Down, Left, Right, Left, Right, B, A.

Defeating Pyro
Jump on the wall, then jump to the top. When Pyro shoots the fire circling around him, drop just before it hits the wall then dash to him. When he shoots out the birds, stay near the wall. When they are about to hit you, double jump over and slide below them. Repeat this until they burst.