Fire Storm

Fire Storm (ファイヤーストーム) 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.

Damage Data Chart
Known damage values for the original Mega Man. 

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 No. 1: Mega Man
 * Hyper Storm. H: Mega Man: The Wily Wars
 * Wily Machine (2nd Form): Mega Man: The Wily Wars
 * Oil Man: Mega Man: Powered Up
 * Yellow Devil: Mega Man: Powered Up

Bosses Immune To Fire Storm

 * Bubble Man: Mega Man: Dr. Wily's Revenge
 * Enker: Mega Man: Dr. Wily's Revenge
 * Heat Man: Mega Man: Dr. Wily's Revenge
 * Wily Machine (2nd Form): Mega Man: Dr. Wily's Revenge
 * Fire Snakey: Mega Man: The Wily Wars

Trivia

 * Mega Man is seen using the Fire Storm against Ice Man in the intro to the cartoon, using it to simultaneously blow him away and set his parka on fire, also defrosting a frozen area of the city. However, in the games, Ice Man's weapon is actually Fire Man's weakness.
 * In Mega Man Megamix, in Napalm Man's profile, it states that the scattering explosion produced by Napalm Bomb is called a Fire Storm.