Boss - Metal Man

Damage Table

Values are for "Difficult". Double them for "Normal"

Strat

Metal Man is programmed to jump and attack if one of these three scenarios is true

1. The player presses (but not is already holding) B
2. the player crosses the center of the room
3. a few seconds have passed without either of situations (1) or (2) above happening

As such, the easiest way to attack Metal Man is to enter the room with Quick Boomerangs equipped and already holding B. As long as you do not cross the middle of the room, Metal Man will just stand there....almost.

Difficult vs. Normal

On Normal, you can just go in with this technique and defeat Metal Man with 4 boomerangs. On difficult, you can only land 6 hits before Metal Man jumps and attacks anyway.

You can accept this and just hit Metal Man after he lands, but there is a faster strat that makes Metal Man jump early and then never again.

Refights

Metal Man is famously weak to his own weapon - on Normal Difficulty it one shots him.

Jumping in place and firing three Metal Blades is typically enough to finish the refight. Just remember to move off the teleporter if you want the health so you are not immediately beamed out.