You can't make an AoE spell specifically target. You always have to base your spell off a spell with the same type. This means that Single target go with Single target, AoE with AoE, Channeled with Channeled and so on. So you can't do it, only through object editor, and make it look good as well.
In order to make this work, you will have to trigger the effect of the spell yourself. You can of course use the AoE spell with very very small range and cast it at position of the (Target Unit of Ability Being cast) with a dummy unit. However, this is risky and may get more than one unit involved in its effect. So yeah, I suggest triggering the effect if possible.