Metal Man's room has a conveyor belt which changes its direction on intervals. When the conveyor belt moves to the left, you can shoot him from afar. Evade his Metal Blades as he attacks whenever you shoot at him. He attacks one blade per shot, so firing consecutively is not advised. Then, when the conveyor belt switch directions, move near him & shoot so he'll jump over you. Then go with the flow of the conveyor belt & shoot him again from afar. Another pattern Metal Man does is that he jumps just behind you. When he does this, do a short jump-shot & he will either jump behind you or jump far away from you.
Metal Man is usually the first Robot Master to be battled, as that is the most strategic. This is because the Metal Blade is extremely versatile, powerful, and slow to run out (of ammo.)
Metal Man is weak against Quick Boomerang, (But is even more weak to his own weapon, Metal Blade.) which you will get if you defeat Quick Man beforehand. The Quick Boomerang deals 4 points of damage against Metal Man, but you need to move near him as the weapon has a short range.
In Dr. Wily Stage 4, you will encounter Metal Man again for a rematch. This time, there is no conveyor belt, so you can snipe him & evade his attacks from afar. Or you can attack him with Quick Boomerang, but you need to move near him.