Just kill his focus, he'll do the rest. You can use dmg multipliers at this stage. However, near the end of the fight he stops killing himself soo you'll have to attack him.
Bio--easiest, just put on 2 disrupts and spam them on him, not sure if it requires focus or not to disrupt him in the first place. Hydro--easy but long, have veradux and roald on defensive and just do 2 flash freezes with a regulate in between each, then mind freezes and regulates. He will be electric stormign you for more focus though, so it's rather long. Psycho--goes down easy if you spam DoT on him, use haunt and traumatize...by spam DoT I mean implosion if you can, it'll massively boost the damage he does to himself (disregard if you can't get implosion without overdrive).
Also, if you get him to 0 focus and he decides to use deep burning on himself while holy scars is on, he'll pretty much kill himself.
I play the "hysical" class. Self heals up the ass and more hitpoints than you care for. I also grind with the "Training" levels, so I'm very often a few levels above whatever enemy I'm facing.