These things have a TON of hp. Trioed a couple of these last night with a 68 mage, 66 chanter and 66 cleric. Didn't have that hard of a time getting slow to stick after tash/malo combo, but it was still taking down the pet faster than we cared for. Even stacking on the dots and nuking a good bit these things still took long enough to require re-slowing. If I had to guess I'd say these have around 80K-100K hp.
Oh yea, and watch out for the enrage at 10%.