So, I reached him with 2 characters and both times, the boss just suddenly insta-died after a while. I assume he just goes into some crevice where he doesn't fit, clips the terrain and gets killed instantly.

Did anyone actually manage to kill him by shooting his health down to 0? Makes the game kinda anti-climactic.

Hi IdbeholdME,
Actually yes. I did this. Probably you had experienced some wired bug. I have noticed myself being stuck couple of times between some polygons (surfaces?), so your idea might be very much true. Maybe reinstalling the game helps?