Well Drain is apart of her being a DPS, especially since Drain has a shorter recharge time compared to Lightning.
What makes Speed so powerful on the Dark Mistress is the fact that she can escape enemies very easily. She already has a fast movement speed and she only has two spells to cast so she doesn't stop very often to cast spells.
She also has a lot of health (700, same as the Barbarian's) for a support unit. She also has a fairly high Armor stat, (50) so she can take a hit and doesn't get hit very often. (especially with her 70 Defense)
So I think maybe decrease her movement speed slightly, lower her health down o 600, and set her Armor stat should be 40, then she won't be so powerful.