Theres a broken ship on the higher ledge with traps and a chest near the area Mordus is located. You can teleport him there near the ship hatch with increased spell range and a teleport spell, he'll be stuck there the whole fight unable to do anything and you can just teleport him back when you please. Also this is not the only case where there are places in the game where you can just teleport people on platforms that they cant get out of and you can cheese trough them.
One could argue that you could just lower the difficulty if you need to resort to cheesing but.. just wanted to you guys to know.
https://puu.sh/xObs2/904c2c7692.jpg