Mega Man 6 (USA) [Hack by Dragon Eye Studios v1.0] (~Megaman Showdown VI) (Boss Mode) is a ROM hack of Mega Man 6. Dragon Eye Studios significantly altered the original game, creating a 'Boss Rush' mode which focuses solely on challenging boss battles.  The original Mega Man 6 gameplay is retained, but restructured entirely around the boss encounters.  New challenges and difficulty adjustments were likely implemented. The overall gameplay experience is vastly different from the original, focusing on rapid succession of boss fights, testing the player's skill and mastery of Mega Man's weapons.  This is a fan-made modification and not an official Capcom release. 

Mega Man 6 (USA) [Hack by Dragon Eye Studios v1.0] (~Megaman Showdown VI) (Boss Mode)

Mega Man 5: Protoman Edition