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1) Batman lures Doomsday to the deserted island where the Kryptonite spear is that can kill it. He formulates and shares that plan to use the spear to kill Doomsday.
2) Batman waits until the critical moment to fire his last remaining Kryptonite mist grenade at Doomsday. Doomsday inhaling the dust may well have proven decisive in that battle. We’ll never really know although maybe one day if someone asks Zack Snyder about it I’m sure the answer will be, yes, it made Doomsday just vulnerable enough to be killed, etc.
Superman provided the muscle and the sacrifice of his own life. Wonder Woman weakened Doomsday first with Godkiller with the cut to the Achilles and by holding him with the Lasso of Truth. And Batman‘s contributions are already noted. It was a team effort for sure.