The Dark Mage is a ranged support enemy type. It has decent health and deals a moderate amount of damage. They spend most of their time summoning skeletons from fallen allies, healing its allies, and occasionally shooting magic bolts at heroes or defenses.
Like other ranged enemies, placing defenses that have long range or placing traps and auras in the lane will kill them before they get into range to attack. If you have Series Ev2 you can use her Reflect Beam defense in front of a blockade to bounce the projectiles away. They die relatively easy to any damage done to them.