Well that spell is realy simple, i try a short one.. (iam sorry if there is a little error but iam creating it ouf of my mind^^)
Condition:
A unit start the effect of an ability
Condition:
Ability being cast = Your Spell
Action:
If/then/else
Condition: Boolean (If target unit of ability being cast an ally of casting unit)
Then Do:
Set life of targeted unit to life of targeted unit + 100
Else:
Damage target unit of ability being cast by cast by casting unit, for 100 damage points use a magic blablabla
Thats it, i hope!