all the mobs around Kalendra's Coven, inside and outside, do way too much damage. It's not just 20% more or 50%, it's more like double the damage for any other mob with the same amounts of dots.
you practally have to chainheal a same 19lvl a fully buffed paladin and still 2 mobs can spike kill you. 1500 dmg (900 strike attack + 600 normal) is a bit much on this lvl, in a small dungeon, from non-named mobs.