During the fight, Cultist uses a teleportation spell which, if you do not move from the area of effect he grasps you in claws, pulls you into the ground and moves you to another area of the arena. During this move, he teleported me outside of the arena play area and I got stuck behind the magic barrier in the bottom left of the arena. I was unable to move back into the arena and died fast due to inability to avoid any of his attacks which were still able to hit me.

Edit During the second attempt I also found that just after he casts the frost shields that rotate before being drawn to him, if you run and stand top left on the golden platform (cirlce left of the ancient) then after his second attack (The lightning blast ) when he trys to teleport near you he bugs and just stands facing away from you only re-engaging you if you move closer to him.