Its a bit frustrating at first, but its not too bad once you get the pattern down. I use voidlock, with child of the gods/nezarecs sin (for weaken and devour). Graviton lance, with an auto loading fusion in top slot and fallen guillotine (relentless/whirlwind). Triple void surge, void siphon/firepower/reaper. Devour on orb pickup is also really helpful so when you run with the mote you can pickup for life. Once I pop the last mote, I grenade to weaken the boss and nova to clear most of the adds and do boss damage. Then pop rift at the boss (create voidsoul for continuous weaken). Then start to slash away at the boss. I do mostly fast attack (about 10x attacks) until goblins spawn, then I should have a grenade back (Nezarecs sin will buff ability regen, and by the time you start to work the boss, it should be 16s so grenade will come up quick) and throw that close enough to the boss between a goblin so that it kills the goblin and weakens the boss. Grab the orbs for devour/armor mods Continue to slash away 5 fast to 1 heavy attack. Rift again for another voidsoul, kill goblins as needed to get another grenade while devour is up. I can usually one phase him solo with this method, if not, I play it safe and the nova to start the second damage phase will finish him off. Well is typically easier for the damage phase, since you dont get immunity from the shield. Without it, you need to rotate to the non shielded side when slicing him up. Graviton Lance will clear out most of the psions by shooting one and letting cosmology do its thing. If running solar, try sunshot for similar effect. Using a weapon where you need to kill each psion individually is not effective.