Air Man fighting Snake Man is pretty random, but Snake Man's stage has always contained a rather incongruous air-based section that is perfect for him.Wood Man fights Spark Man because wood is nonconductive.Crash Man fights Hard Man because Hard Man's whole thing is meant to be extreme durability, and what better to handle that than a bomb? Also, Hard Man has a pretty predictable and static pattern, which is the perfect thing for a character who uses slow timebombs.Top Man beats Shadow Man, and so does the speedster. Also, both Quick Man and Top Man could be considered 'speed' types. Quick Man fights Shadow Man because the Quick Boomerang beats Metal Man, and Shadow Man (and his weapon) is pretty similar to Metal Man.Bubble Man's weapon is the only thing that can damage the hologram projector in the fight against Wily in Megaman 2. Bubble Man fights Gemini Man because Gemini Man uses his twin as an illusion not unlike a hologram.Everyone knows what happens when you hit Metal Man with his own weapon, so it's not too far of a stretch to assume the same thing would happen with Needle Man's weapon.