Figured this was a harmless enough thread to ping you in. 
Didn’t wanna bump a year old thread:
Not sure if you figured it out, but you can solo this now! Just kill the mini-robot as usual, TAG THE MAIN BOSS, and immediately run as fast as you can back to the entrance of the dungeon. His annihilation ray will miss you.
Provided you hit him (he’ll be immune so it wont do any damage obvi), he won’t evade and will chase you. Run back as fast as you can and do the channeling on the pillar, which will allow you to overcharge him and kill him, as he just hovers there the entire time looking at you. Lmao.