once you teleport into his tower, don't move. He has big aggro range, wait for your group to be ready.
step on the lift, and take him down to 50%, but make sure your big-hitting DPS is ready for later! the lift will then ascend to the top - unleash hell now!
On top of the tower, directly target the boss, and make sure tank grabs all adds that may pop. Burn him down.
Note the following:
You must interrupt DISSOLVING SPIRIT! It's a death-touch instant kill. This can occur during the first or second 50%.
Horns of War is a big AOE that can not be avoided - healer must ensure group wards/reactives are up or heals are ready to go
When he casts Temporal Soulleach (Spelling), he will create a red cone in front of him. Tank needs to move out of the red circle and behind the mob. Moving to the side will not always work it appears. No one else should obviously be in the red area either. He will insta-kill anything in that range. AFter it hits tank repositions back on the other side so people can get his back.
Best position for everyone other than tank is not far behind the mob, even the healers. Every once and a while a group member will get an uncurable elemental. If that person gets it back far away from everyone else if you have it or it will damage and power drain the rest of the group.