Ignore everything except for mirror and dps like crazy. Soon as mirror ends hit auto follow MORE THAN ONCE. He will stun one guy so you have to keep hitting it.
DPS DPS, stop for mirror, DPS DPS.
When he gets to about 30% (depending on your dps) have all your guys on auto follow. Be ready to move as soon as you see warning. You can run THROUGH him and if your guys are above 20k they should live if you moved quickly enough (even better would be to hit forward on all guys). It's crucial that you are all in melee range of him though as it doesnt give you much warning. Once you get to other side of him dps the crap out of him.

Keep doing that.

If moving through him doesnt work for you there is always strafe sideways and then autofollow. Best to always keep within melee range of the boss, further back and his AoE moves faster than you can run, simple math.