well, not sure what you're after EXACTLY...but...
i know you probably have to make a dummy unit, it usually has locust so it doesn't get bothered by other units or players. it usually is invulnerable and invisible and might have infinite mana and regen or just uses spells that cost nothing. cooldowns for spells are also usually zero.
anyway, you take the dummy unit in the trigger and make him cast the spell.
it can actually be really complicated. i tried making a spell and i thought it would be so easy but apparently it was the culprit making my map crash. i finally had to have someone else do the spell for me.