The 1.4m HP is not really the issue, it's going to be the fear because he hits really hard and the instant your tank gets feared and turns his back he's going to die.
I have 6-8 manned him (not boxing though) with a single tank at 70, I don't know if a single tank could hold all 4 WMs, dogs, and Drek anymore. At L80 I was recently critted for ~32000 on my Warlock -- through SL and ~900 resil -- by the whirlwind from a 4-tower Drek'thar. I don't think he hit this hard back at L70 percentage-wise anyhow. Don't forget all the WMs also cleave / WW so any sort of melee DPS is going to be out of the question. And the chain lightning is going to fry ranged if he casts it on them.

It would be much easier to do with 2-3 tanks, 2-3 heals and some ranged DPS, as long as you're geared. Any melee DPS should probably stay at the keep door and stop people from getting in. For a 5 box group... 2 tanks 2 heals 1 dps, if you're really geared it is probably doable. 2 holy paladins is what I'd take for healers in this case.