I found a way t0o do it, as soon as he summons his 2 minions i circled towards his right side and dps him with my pistols from max range, he never moved so i took him down in like 10 mins or so. Thanks a lot everyone for the support.
I had the same problem, and your solution worked perfectly. When the boss creates clones, it's best to go to the left wall (left if you're facing boss) and then go up and start attacking him from the flank. Boss won't change the phase, he will be stuck there casting spells. Killing him this way will allows you to avoid the blocker bug where you wouldn't be able to progress further.