Hmmm.
Yes, this is tricky.
I was thinking of Part 3 being a bloody mess, fires in the background, and me screaming at one remaining (somewhat bloodied) fangirl that I WOULD kill them the next time they tried this.
And GLaDOS huddled up near the ceiling going "oh god oh god oh god" or something.
I haven't decided, but it seems sufficiently climatic and funny at the same time...