Some people used to try to Line of Sight him.
Basically fight near the entrance area, either just inside or just outside of it.
Position him, so he's on one side of the entryway.
When summoned, run to the other side, and use the wall to Line of Sight the blast.

Crude diagram:


..... Boss.. Tank....
============= WALL ===............. ======== WALL =======
.............Run Team Here.......
.............When he AoE's.......

Felguard Area.............................................. ....... Ramp you came down....





It's probably easier to just run through him.
Or to strafe with Q or E.

Backing up will not be fast enough, you only move half speed backwards.