Oh my god, what an annoying boss!

I tried the method of having one of my clones walk into melee range (and made sure she was standing closest and meleeing him) but out of about 20-30 sleeps dished out over the course of 4 wipes only one went onto this sacrificial mage, the rest were aimed at the paladin.

In the end I downed him by using a little trick I save up for hard situations: Pyrobast from max range, followed up by a fireball while beginning to build agro on my tank. As soon as the fireball was cast I iceblocked the 3 mages and continued building agro on the pala. I may have just been lucky, but I didnt get sleeped until after the mages ice blocks had worn off and they had snapped off another 2 fire balls. By that stage he was on about 5% and on fire went down super fast.