It's nice to know that even after almost two years P.A.X. is still as glitchy as he was at Launch. I had started a new save for giggles and to get the hardcore boss weapons. I took my sweet time looting about Abandoned Production and when I finally decided to fight P.A.X. Now before anyone comments on it. I've beaten P.A.X. before so I know how to win. The thing I forgot was the dumb hit box glitch that makes his frakkin legs immortal so that he won't fall even when you've beat the crap out of him for over 10 minutes. Granted after the first 5 minutes I should have realized something was wrong even if I was taking my time to make sure I got the hardcore kill. Now that I've hit the end of my rant my question for the Devs is: I thought this was patched? What the heck? Thank you for your time and letting me rant.
I see this confusion about the PAX fight often. If you want the hardcore kill, you first have to destroy the armor plates on its legs. When its damaged enough, you have to trigger it using the stomp, quickly attack the other leg while one is in the air and it will fall down.
Took me a lot of time to realize that but it makes sense.