It depends on your strat, but focus-fire classes (rogues, hunters) should assist his target first. If you get to the 50% mark and his target is still up (for some unknown reason), have one of your real resist tanks taunt it off him. Edit: Oh yea, the usual demo shout and thunderclap debuffs should also be up on his add.
We also put both tanks in the same group so they receive both auras/totems and both shouts. Imp defensive stance also helps quite a bit here. Your "off" Hydross tank is just taking up space if he's not picking up adds. Our dps in resist gear is even worse than our dps in real tanking gear.
I am still hurt by the revoke of the DW Devastate change