Charizard's Side Special move is called Rock Smash. It produces a boulder and bashes its head into it, which somehow damages the OTHER guy. It does incredible damage, which is good against bosses.

After beating Boss Battles on Intense difficulty a few times using this technique, I found that it wasn't really all that difficult once you get the hang of it. My newfound confidence led me to become a bit sloppy during this recording, but I won anyway.

Just goes to show that you can mess up and still win on this one.