Try this:
When Metal launches crystals at you (Big clear blue things), Use homing attack on them. For each succesful hit, your Team Blast gauge shoots up. He'll Scream "Take This" when he uses this attack, so that's your warning to switch to Speed Formation.
When Metal is about to use a spike attact (A lot of small-ish spikes flying everywhere, then coming to you) Use Thundershoot. The guage doesn't shoot up like the Crystal attacks, but there are so many, it should raise significantly. He'll Scream "I'll Skewer You" when he uses this attack, so that's your warning to switch to Fly Formation.
When Metal is about to throw a ship at you (the ones from Egg Fleet), Use Knuckles's (self-called) Nova punches. The one you can repeat over and over. The guage shoots up by nearly half (or more?) if you destroy the ship. He'll dive down to retrieve a ship before using this attack, so that's your warning to switch to Power Formation.
Now, it takes 5 team blasts to beat him. So, just repeat as stated above until you beat him.