Damage Taken From Elements
When he uses poison bomb he will poison himself but he is not weak or strong against earth damage. Thul will run at about 40 health (tested at 37, note that Quara Constrictors run at 40). Thul will retreat at 40 (1.36%, deep red) health.
Thul can be found in two areas: he is in the quest room when someone enters it and he can spawn just west of the teleport to it. Thul and his Massive Water Elemental summons do not cross energy fields unless they cannot reach you otherwise. It can be handy for paladins and mages to throw an energy wall toward the teleport (quest room Thul) and run over it to avoid close-combat attacks (they will all run around the coral). Note that Thul can poison bomb over energy walls, but will do so very rarely while chasing you around them.
Thul's loot has drastically been improved in the 9.2 Update to make him a more desirable boss to hunt.