Let me tell you how we do it in a 7+2+1, 2 healers and 2 tanks:

Start:
My 7 on thane (1 healer+5 dps+1 tank)
The 2 (tank+healer) they pick Rivendare and the Holy boss and they tank it near the holy boss
The 1 - yet very undergeared (lots of greenies) arms warrior gets at max range of the shadow boss, close enough to my healer to get healed and just dances to avoid zones

My 7 usually kill Thane very close to 3rd mark, then they move to the holy boss corner
We can heal through 4-5 marks
When I get near, the 2 switch with the warrior, I taunt Rivendare and I can usually kill him before the 4th mark

We just then switch (around 4th mark) till the last two are down. Our warrior usually dies ^^. But he'll get his revenge soon