I tried setting up a map that i think had the same predicament that youre having..... and i think i have the perfect solution, although it is quite complex.
Lets overview-
Your Problem- You need a way to deal damage.
Solutions-
Use the traditional... Move in like 6 units that deal an amount of damage, and then remove those damagers.
-Lets face it, although used alot, this version is tacky.
Use a sort of traditional... Move the SpellCaster's Opponent to an area where there are unit damagers...
- Also tacky, not as much as the other, but still.. hes dissapearing for a while... not cool.
A third option involves Grids and burrowed units.... If you dont understand my description, ill just either whip up another Smiley Diagram or throw a sample map at ya.
Yes, it is very effective, but hard and tedious to put into place.
Now, lets say.... your spell caster hero has 1000 HP.
Lets ALSO say that you have Lings, and they are 10 HP each.
Ok, so you have 100 Lings.
Constantly have the burrowed 100 lings be centered under the hero, whilst the hero itself being invincible.
The Lings will keep the HP...
However, you have a problem now, dont you? Whenever you have burrrowed units constantly being centered under a unit, It is really really really really really slow... how do we fix this?
We dont have it under the hero.
By setting up a grid that moves the lings down... about half of a location square underneat the hero, you can have the lings act as the Hero's HP.
For the spell casting...
Lets say you want the Spell to do 50 damage?
(Do whatever effect,... Exploding Scouts... blabla bla, and then Kill 5 Lings under the Opponent Hero.)
Let me know how it works out.
Oh, and vrael... Copy me again, and ill punch you in the nose. lol
None.