I finally beat Boss Battles on Intense!

Marth is defintely the way to go if you can dodge Tabuu's Off-wave. It's very easy to beat the other bosses if you're good at Counter Attack. The only hard bosses (aside from Tabuu) are Duon and Porky, the latter being the hardest because the missles that Duon fires do an extra bit of damage when you use Counter Attack.