I'm getting even closer to something resembling a win, but that still wasn't enough.
That was me collapsing the middle pillar onto the boss, but maybe that was a mistake and I should've went for the side pillars instead. I also brought Spot along for some additinal healing, but that was also not helput because he died in about 2 fireball volleys. So I'm thinking to switch him out for an additional ranged damage operator like... yeah, I don't have one.