I believe Xun has a good idea regarding headache, but I think it would make more sense for the spell to be only moderately difficult to land, reduce a moderate amount of mana per tick, but last 3-6 ticks and not be stackable. It may even make sense to be followed by a cool-down period, depending on its effectiveness in reducing mana.
In my experience energy drain is most definitely not as easy to land as blind, and as Kiirion pointed out, the cost vs. average times of casting to land renders it ineffective. It's a catch 22 because while it seems impossible to land, reducing 50% of your opponents mana at the beginning of a fight is incredible if they are a pure spell caster.
“Waste no more time arguing about what a good man should be. Be one.”
~ Marcus Aurelius