Assassinate the Assassins Glitch

I can’t help but love Borderlands 2. To level up, I went to do a few side missions, including Assassinate the Assassins. However, when I got onto the third assassin (I believe Assassin Reeth), I attempted to do the melee takedown in order to complete the additional objective, and thus I died. The previous way I entered the area is now locked, even if I eliminate all of the enemies in the area. I tried going to the area directly above it, he didn’t spawn there either. This really sucks, because I don’t want to restart my campaign (I’ll probably just leave it until my Vault Hunter playthrough). I’m in the process of finding someone to do the mission with, but this should really be fixed. I’ve tried all of the regular solutions, leaving the area etc., but he will not respawn. The gate always closes right before I manage to get in. Any ideas? (btw I found some forum posts dating back to 2013, so this is a pretty old glitch).

Edit: I have indeed opened a support ticket, but the automated email I received recommended that I posted my problem here. I haven’t got a response yet.

Thread is a dupe, feel free to lock.