Hello, you can test number of mobs alive like:
return wManager.Wow.ObjectManager.ObjectManager.Me.Position.DistanceTo(new Vector3(1, 2, 3)) < 30 && wManager.Wow.ObjectManager.ObjectManager.GetWoWUnitByEntry(1234).Count(u => u.IsValid && u.IsAlive) <= 0;
(replace "1, 2, 3" by mob position and "1234" by the mob entry id)