Trick famosissimo
A glitch in the original release of the first game allows the player to quickly defeat Yellow Devil by shooting it with the Thunder Beam and repeatedly tapping the pause button when it makes contact. Every time pause is toggled it will register a hit. This actually works with any boss, and even the player, as it really is just the invincibility counter still expiring while the game is paused. A similar glitch can be performed to dodge Boobeam Trap's attack with the weapon select menu in Mega Man 2.