Psionics are OP. Stacking Crit damage is OP. Together you get to 1shot the boss. I never even attempted the mutagen puzzle as I stumbled into the final boss room while exploring before going back to my loot pile to bring the mutagens I had collected.
For anyone wondering how, I had 16 will and an effective thought control skill of 305 with my headband and psi beetle armor. My crit damage bonus was 287% with the feat, headband, and trance drug. With this it was just a matter of first using mental breakdown to double the effectiveness of the next thought control ability and then forcing a crit with psionic mania before using neural overload.
I've also seen the post of the guy who did 5k damage. He mentioned that the crit damage bonus of one of the unique melee weapons applied to psionic abilities but he never answered which one it was and I couldn't figure it out myself or maybe I just never found it. I think he was probably also using toxic gas grenades for the contaminated debuff.