His ability is called something like crushing dark, watch for it on the cast bar, you can either interrupt it, or use the environment to break his line of sight, this works and was the only way i could solo him, keep distance, use your stun only as last resort, keep your interrupt button smoking, and if you're running a healer like i was, keep your shield and instant hot rolling, also use your instant cast slow to keep him off you. Took maybe 2 full minutes to kill him without my companion as he kept getting one shot.