Spell help!

Anybody knows how to make this spell ? E.g. A unit uses a spell on an enemy unit. Dealing 70% of the triggering unit's health to become damage to the targeted unit, making the triggering unit's health to 30% left. Anybody knows how .. ? please share ..

I did not understand if you want the casting or the target unit to be with 30% life but, you should use this trigger anyway:
Events
Unit - A unit Starts the effect of an ability
Conditions
(Ability being cast) Equal to (Your Spell)
Actions
Unit - Set life of (Target unit of ability being cast) to 30.00%
Just change to casting unit if you want.

Oh yeah thanks. But i wanted to know whether i could make a trigger about using the 70% loss of health of the casting unit to become actual damage done to the targeted unit. I was asking that ..

actually, it would be better if you took off 70% of thier remaining health. cause lets say a unit with 100 health has 10, if you cast the spell on him he would have 30.
so get 30% of the units current life.