A good strat for this mob, is your tank ONLY attacks him until the first spawn. This will save you a whole lot of trouble ping-ponging agro after he steals all the tanks power (a nasty ability).
Also when using fellowship abilities use the Blue icon to help regen a little power during the fight.
Everyone else must agro the spawns that come every now and then and kill them quickly (ensuring the healer stays safe). Watch the main tanks health when boss is getting low on health, as his damage output seems to increase.
If you loose the tank near the end, just go all out on the boss, when he dies he takes his spawns with him.
If all goes horribly wrong just run outa the room back to the next to reset the boss.
Good luck