It's alot harder on heroic, at least the second boss! 4 shamans:

Snake boss:
I place totems, hug the tank, pop bl+fire elementals .. the fire elementals will deal with the snakes and you just nuke! He might cast Poison Nova, then you run away! You and you tank run in different positions.

Second boss:
It's ALOT more tricky, alot of aoe is going on here, I spread the shamans out in a halfcircle and use my forward/backward keybindings to avoid small aoe spots on the ground, which the water elemental spawns. The tank tanks the mob in the middle.

Place healing streams and be ready to heal yourself ALOT!

Mammoth boss:
Just tank and spank really, but heal yourself up after "Quake" (does around 75% dmg), you can try to do the achievement where you have to interrupt the mammoth phase, and kill him before he gets to cast it - that way you'll avoid "Quake".

Extra boss:
Tank and spank, do not stand infront of the boss, cause of the spit (does 60%) which he does fairly often.

Last boss:
Be sure to interrupt follow, cause you'll lose alot of dps/healing, if the target that your slaves are following gets the knockback. In second phase, be READY to heal the tank, he does 50% hits on a Naxx tank. Remember to heal the target, that he "targets".

EDIT: I don't have alot of time, so just wrote this in a hurry, sorry for the names! I think that second boss is the hardest, but I've only wiped once in 3 clears.