Casting Spells on Units

I want to be able to cast a custom spell on a unit every 5 seconds, I got the timer part down but I don't see how I can cast the spell on the unit, please help.

if you wanted it to look like the spell was being cast w/o a caster, u can:

1. create a unit possessing the desired spell

2. hide that unit.

3. order that unit to cast the spell.

i think that ought to work

Yeah I've tried that but how do I order the unit to cast the spell? I didn't find anything in triggers.