Black Robed Witch Helena is a cost 100 Dark mage damage dealer with an arguably "complete" set of skills. She's able to deal high damage to just about any enemy, even against tanks. Cast speed is fast, and has instant area attacks + heals from her sub job. With the right setup, she can definitely be an unstoppable force that will last for quite a long time.
You'd have a tough time building against her since, as a unit, she has some good magic resistance penetration through her master ability + the farmable weapon, Cypress Pile, and a lot more if she uses Peacetime Brooch, but she's also able to destroy magic barriers (Devastation), decrease SPR (Witch's Hex), decrease dark resistance (Limit Burst), and a guaranteed hit ability, Black Rose Judgment.
Her AGI is considered fast against other mages, but far from the fast melee units, so bursting her down faster before she gets to you is an option. However, Black Witch Helena does have her own special VC, Black Rose of the Battlefield, to bump her AGI quite high. Her casting speed is also very fast because of her passive Witch Mastery at EX, and Darkened Focus to reduce casting time. Even if she doesn't use them, her Arithmetician skills are all instant area attacks with good range, AND there are also 2 heal skills available.
She's very strong without her EX, but going for the EX can make a pretty big difference as well. The casting speed increase from her passive can help kill your opponents before they get to you, Witch's Hex will be able to reduce the enemy's AP, and job lvl 25 ability for the guaranteed hit ability so she can handle all types of enemies by herself.
Thankful that she is not time limited. Although I want to get her ASAP when released, I don't mind waiting so I can save my vis on certain time limited units