First time I cleared it with a minute left. The other two times were around 4 minutes. You really have to burn the boss from the get go once the lift reaches the platform -- having everyone on the same element in the burn phase helps a lot. On the last attempt we pushed him so hard with arc supers + Coldheart we got him immediately down to about 1/3 health and he completely skipped the jumping on the tower part. But since we weren't trying to do Rat King we goofed around for a bit and wasted some time before killing him. If we had set out to do Rat King I bet we could do it if we optimized that last battle better.