Or, you could just modify the Targets field in an existing ability, make it affect only enemies and neutral, then add it to a dummy unit.
Run a trigger that summons the dummy unit when you cast a spell, sets the dummy unit's ability to the level of the ability being cast, and order the dummy to use the ability. Add an expiration timer to the dummy.
If it has to be Strength based, then yeah, do what Need_02 says...