Unfathomable Victory

Not sure about that but you can use pets to help. In my case I just spammed party heal and buffs.

yea, but if golems work, it's easier to alchemists too, and not just restricted to bards and clerics.

Golems absolutely work. I've done this several times, using the Golem's AI, and the UI where you become the golem. You just have to be careful not to accidentally attack the Girgashy when you're inevitably knocked out of the control UI by Mineral Hail or whatever else.

(Had to edit, sorry.)

What about the player character hitting only Holy Confinement and Soul Absorption and not attacking/splashing Girgashiy?

Hitting crystals doesn't count as damage for the quest. I would think souls wouldn't either.

Tested with both Colossus and Pierrot Marionettes, a single hit from them plus the remainder of the raid while on my Windmilling Bone Dragon gave me the clear.