"So, let's take a master sharpshooter, a crazy Australian with a fondness for knives, boomerangs, and boomerang knives, Harley Quinn, and a giant shark and have them kill the Justice League."
And nobody at Rocksteady or WB thought this was a terrible idea? You could have easily kept Braniac around and made it so these guys were on a suicide mission into his secret lair while the Justice League is out there fighting off his armies. We've got
The Dirty Dozen after all. Just treat them like the psychopaths they are and have them constantly bicker and snipe at each other, Deadshot being the pragmatic team leader who really doesn't want to piss off Waller but hates who he is saddled with, Boomerang there because he wants to start knoifing people, Harley for the same reason, and King Shark all "Hurr durr I want a steak".