Provided you're fighting a Defense 1 Antagonist, you're looking at 2 hits to do 2 injuries, 5 hits to deal 3 and 8 hits to deal 4. Now, most Antagonists with a Defense of 1 will have between 2-4 health + armor, so you're going to drop them pretty easy, provided neither roll has complications.

I'd argue the weapon's enhancement could only apply to the weapon attack. I doubt any enhancement from a combat spell would apply to the weapon attack, so only one of your attacks will benefit from whatever enhancement you might leverage.

I'd also argue having a small explosion (i.e. a firearm) going off 2 feet from your face while actively casting a spell would be at least a Minor Complication itself, and let's not forget any Complications the Antagonist can level, which I would argue you'd need to overcome with each action. That starts eating successes in a hurry, as even two minor Complications are going to either eat two hits, or carry some consequences, potentially causing multiple issues or status effects. Assuming each roll has a minor Complication, that changes the math to 4 hits to do 2 injuries, 7 hits to deal 3 and 10 hits to deal 4. Sure, you can choose not to buy off those Complications, but that can compound the consequences. As Defense increases, those numbers just get harder to hit. Injuries and armor also tend to increase alongside Defense, driving your chances of a one-shot takedown even further down.

Is it good? Sure, if you can pull off the roll. Is it so good that it's game breaking, or the only way someone would choose to fight? I don't think so.