heal or holy light does not even start casting when targetted on a unit with full health. Therefor, it cannot be used on units with 100% health.
Since healing a unit already on full health is utterly useless, I suspect you want to use holy light as a dummy. But then, there are plenty of other spells you could use: shadow strike (no stun) or one of the various stun spells such as fireball, thunder bolt, etc. etc.