I've done the encounter a few times on normal and hard.
Last night after 4 tries on the final boss, we gave up. We've done it before without any issue.
What would happen is the final stage would trigger (Mentor's green beam & bombs etc) halfway through the first phase... specifically after the first power core was destroyed.
I guess the question is: is the encounter bugged for anyone else? It never did that on hardmode for us before, so i wonder if we should just avoid it or try it again?
Its always done that for us, he only does it for a little while though, not like the end where he does it until you kill him. We get it every single time a power core is destroyed.