Put a +1/+1 counter on target creature you control, then it deals damage equal to its power to target creature an opponent controls.
If either creature is an illegal target as Knockout Maneuver resolves, the creature you control won’t deal damage. If the creature you control is an illegal target, you won’t put a +1/+1 counter on it even if it’s still on the battlefield.
You can’t cast Knockout Maneuver unless you choose a creature you control and a creature an opponent controls as targets.