Did You Know Gaming? Forums

Due to an oversight in programming, Metal Man is weak to Mega Man's standard buster in Mega Man II. In addition, he is weak to his own Metal Blades as well, the Robot Master only able to survive one or two (depending on difficulty) hits before being defeated (testable during the boss rush mode during the second to last Wily Stage).

Sources:
http://megaman.wikia.com/wiki/Metal_Man
http://www.mmhp.net/GameHints/MM2-Data.html
http://youtu.be/hG38aamwcpE?t=2m50s

This has already been submitted by email.