I suspect that he could take out atleast half the league over the course of several nights. Lets face it, Batman would NOT let anyone get word back over what was going on. Once they found out it WAS him the rest of the league would likely be in a panic over Batman not only wtfpwning half the league without so much as a superpower to his name, but the fact he undoubtedly did it in an overly brutal, calculated method.. He's also undoubtedly target the original 6 first, to help damage the chain of command and further spread fear and panic.
Given his level of paranoia, as previously noted, he'd undoubtedly have contingency plans for how to take down the big guns and make sure they stayed down. For the lessers.. well, lets face it: who needs a special plan to kill the lessers like Booster Gold? With the amount of effort he's put into plans for taking down Superman alone, imagine the dirt he's got on everyone else.. After all, he's not getting any nicer with age, as the time travel episode showed. You know something's wrong when Batman is having to play GOODCOP with his older self..
Batman is bad@$$. Nuff said.