1 tank
1 melee DPS
3 or 4 ranged dps
2 or 3 healers

People will fight all day whether 2 or 3 healers is better. It depends on your capabailities. 3 healers makes it easier to survive and makes it more possible to continue on if you lose one of them. But you will also take longer and be subject to larger tumult sequences. 5 DPS will end the fight faster but staying alive in the final phase is a bit harder.