Nightbane

[Mirajj]

Let him use Arcane Missiles, it's a cinch to heal through.

In addition to that, all the mana cost on Missles is frontloaded. If he casts even one Arcane missle, he's already spent all the mana he will on it and the only point to interrupting it is if he's beating on someone who can't take the pounding. Focus your interrupts on Fireball/Frostbolt, as the mana is taken from him just before the spell completes. That way he wastes the cast time and doesn't get the spell/mana loss.