Was actually waiting for someone to make a successful roll in case I needed to cast another spell.
Basically, you spend successes on damage versus stunts, after Defense is subtracted from is; one success=one damage, but you can spend successes equal to the opponent's Stamina to knock him down, which is what I'm assuming you want. Since I created a spell pool to subdue him, you also have 2 Enhancements - bonus successes, so you actually have a pool of 6 successes to spend, before Defense is subtracted. You can also Empty the Clip, sacrificing the rest of your ammo for 1 more Enhancement.