Pull him a bit left or right of the entrance, tank him just in front of the large round platform.
First portal phase:
Tank A in red beam, dancing in and out. When I did it, I used 3 styles in the beam, strafed out, 3 styles, back in ... etc.
Tank B or any non mana class in green beam for the whole phase.
Assign two alternating teams of two DDs for the blue beam. When DD A gets to about 25-30 stacks of the debuff switch to DD B.
First banish phase:
You can try and avoid the breath DPSing him, or have everyone simply run to the "long" corner of the room.
Second portal phase:
Tank B in red beam, dancing in and out.
Tank A or any non mana class without debuff in green beam.
Second DD team tanks the blue beam.
Second banish:
Same as before.
Repeat.
That a short description of how we did it most of the time.