He dies alone and his body spawns to the arched entrance. So go back about 50 meters to the arched entrance and you’ll find his body to retrieve the loot.
If you wish i can send a photo.
Now. This happens the first time, the second time he spawns and dies correctly where he has to. So go around for five minutes without exiting the well and come back. Do the fight again and problem solved.
This boss is the easiest skeleton key farming of all exile lands, only the place is rough, not the fight.
Try and let us know, thank you.