I don't know if killing Broly quickly is really an option. When Vegeta went SSG, he was obviously way, way out of then-Broly's league. Even Goku seemed to think the blast vegeta fired at Broly was going to be fatal and begged him to stop. Instead it didn't even scratch him.
The impression I got was that his body instinctively raises its level depending on the danger he's in. That would explain how he never gets hurt or woozy until he's all the way maxed, unlike Goku who takes damage normally between power ups.