Like all Megaman games one boss' weapon slays another. The trick is killing the first one.
Galaxy<Jewel<Plug<Tornado<Magma<Hornet<Splash<Concrete<Galaxy
I recommend starting with Galaxy Man due to his being the easiest boss for me to beat and a very easy stage. The drawback is that Jewel Man's stage coming after that is likely the most difficult of the 8 to survive, though he himself is a cinch with Galaxy Man's weapon. Other good starters would be Concrete Man and Splash Woman.
Oh, and there's no Megabuster in this one, at least not in Mega's quest...