Later on in the fight he will do his AOE knockback twice in a row followed by the wide-range AOE drain (I believe he almost exclusively does this once he starts doing it). If your mages are in a safe range and you get knocked back with the first AOE past them, DO NOT run in immediately as you will most likely get hit by the second one and knocked into the drain range. Run in after the 2nd aoe animation.

Also, make sure Featherfoot is up at all times on all close range DD. It makes a gigantic difference and you will actually avoid quite a lot of damage that way.