That only happened to us when he either enraged or we ****ed up a interrupt/stack. I'll get a vid up later.
Guess the bug is RNG too. We had good enough dps that not once did we see enrage, every single chain lightning got interrupted inside 0.5sec (rough estimate).
We did try stacking tactic only once, but only cause it had no different outcome.
No matter what "cheap" tricks anyone tries, it still is bugged (or just retarded hardmode). I'm happy that atleast someone has got him down properly, it means there's a chance if you just kill him enough times....