This issue is still persisting today on US East Server Frislandia.
Trying to get a group together to 
the boss against the wall, as it appears in the video above, is very difficult. It is hard to reliably recreate the fight and get the kill to progress the epic weapon quest.
Is there anyone here that can update anyone on the status of this bug?
Today i killed him its just you need only enough people thats all. And the agrro ez.
Yep same on Inferni server .
Yep, still broken.
Still broken! Boss does not spawn. Waited 45mins after I assumed he had been killed previously. 
Broken on Phlegethon. MOBs tether is too short and he rubberbands and resets constantly. Almost impossible to do with the size party intended.
It’s been 22 days, can you fix it already? This game looks like a very bad beta from an indie company
Boss is still bugged. Can we get a fix like yesterday please, ffs
Actually just managed to kill it despite the bug. However, it does not give any drops or quest rewards. Been broken for 1 month apparently. I Guess it is impossible to get that 580 sword, ty for wasting 4-5 hours on it.
Tried to kill it for 1 hour now with a 5 man. Boss constantly resets even if he is right on top of his spawn, for no reason at all. Killing the tentacle first also instantly despawns mozrul.
same
I finished the 580 GS sword line and defeated mozrul. It’s hard because he resets but not impossible. Got all the drops and finished the quest when I did. Use heavy attacks to break his stamina and then burn him down with two groups when he kneels.
Killed him easy with a group of 4.
Solution:
Bugged! Please fix!!
Just hit this quest and yea I’m stuck on it too, he keeps resetting
Still bugged, he is resetting after 10-15 seconds
bugged every where…
but i will try this ore solution 
still broken wtf, wasted hours doing the quest line, to get kicked in the nuts, over what should be one of your questing highlights, you worked so hard to reach the grail only to have AGS say fuck you.
Same thing is happening on El Dorado on US West
Still Bugged Can we get some traction on this bug it should be toward the top of the list!!! A Main-ish quest for one of the main weapons sword cmon guys!
We found that if you kill him where he retreats to and stay on that spot, he wont retreat. A tank with threat does well here.