Light looks pretty good for dealing with Tower and dungeon mobs. Her downsides are that she's a squishy Attacker and does nothing but flat damage against bosses, if she even survives long enough for that. And while 40% on 1st skill seems a little low, a CC like Shock is probably worth it.
Current Korean tier list rates Dark as B, and at first I disagreed... but only 40% on a Petrify chance for only 2 turns? That really isn't a great deal. For 40% it should be more like 3 turns, or for 2 turns it should be 60%. At least Thirst is solid.