Spell Help

Alright, I'm trying to make a custom spell that makes the caster immune to spells and drains his mana over time. Basically its immolation with spell immunity instead of AOE damage. My problem is, if I base it on immolation I can get the toggle effect that drains mana, but I can't get the spell immunity into the equation. So I tried basing it on Magic Defense, which gives me the effect I need, but then I can't get a mana drain per second going. Help anyone?

You will have to use triggers. Base if off a dummy mana shield or inferno with no real effect, and add a passive magic immunity ability while the caster has the 'mana shield' buff.