Can't stress the advice to 2 heal this encounter enough. Especially on your first kill. When we first started working on him we were constantly getting the 3rd intermission (which might as well be a death sentence). Once we dropped down to 2 heals and made some adjustments to how DPS were doing things, we got some really great gains! For us (and every group will be different) we blow Heroism in Phase 2 to cut down the amounts of Empowered Whirling Corruptions we had to deal with. Once we got that in line with our other cooldowns it was constant Phase 3s and then cleared. Keep at it, work with your team and enjoy the encounter!
As a good point of reference, your DPS should be doing an average of 200K throughout the entire fight. Tanks are included in that.