By the rules, you can take a spell that hasn't been used yet and turn it into an attack but then it goes away. So, by the rules, it doesn't quite work the way you describe.
It starts with a wall of fire which is keeping the ghouls at bay. Then, on the wizard's turn, he makes the wall explode outwards, hitting the ghouls and dissipating the spell.
I think a compel would work well. I wonder if it would work the same way as an attack. Turning a block into 'Wall of Fire' aspect that gets invoked for effect.
Or, I suppose, create an aspect right off the bat and invoke it as a compel against the ghouls. Basically, they get compelled to stay at bay or, if they choose, take damage unless they overcome the barrier. But then you get into the realm of how many compels can you invoke with a single aspect without spending a FP.