Fire Storm

Fire Storm is Fire Man's power. Fire Storm does double duty--it shoots a fireball, and has a shield effect (albeit for a brief moment). So for the first part of the Wily Battle, you can use Fire Storm to both use the initial fire ball and the shield to do some damage.

Robot Masters And Bosses Weak Against Fire Storm

 * Bomb Man: Mega Man
 * Cut Man: Mega Man and Mega Man: Dr. Wily's Revenge
 * Copy Robot: Mega Man
 * Wily Machine 1: Mega Man
 * Oil Man: Mega Man: Powered Up