Take advantage of their own attributes;
the bosses have a much larger mass so look for attacks that would be awesome if every last hit landed,(i.e. Ike's up+B, Donkey Kong's up+B)
Bosses are easily predictable since they attack in patterns, predictability+counter=win
You can use some bosses attacks against them by deflecting them with fox and co. or absorbing them with Ness and Lucas.
Instead of using really strong attacks, find out which combos have the most DPS (damage per second) and spam that combo so the boss has no time to attack.
[spoilers:3gx9kp3l]HIT THE WEAK SPOT FOR MASSIVE DAMAGE!!!1!!!ONE[/spoilers:3gx9kp3l]
so yeah, if you need help on a specific boss, you'd better ask that, if you're having trouble with all of them in general, I recommend you polish up your skills before attempting intense mode again, work your way up from hard mode.
Try to save your hearts for later, as the bosses get harder by the round. But don't dare go into a battle with over 100% unless you are confident that you won't even get touched, most hits from bosses with this much damage on intense mode mean instant death.