I JUST cleared the 3rd biome late last night on my second go around with the boss. I agree that the fight progresses in a really fun way right up through the third phase.
The whole thing was on a “ok this is the last one tonight I swear” run that I was convinced was totally doomed from the beginning, so I kept expecting to get killed but just kept barely sneaking by room after room until it was way past when I shoudl have went to bed. I stuck with a level 8 Rotgland Lobber throughout pretty much the whole thing. I was passing up level 15 weapons by the end for my trusty little lobber.