Put a +1/+1 counter on target creature you control. Then that creature deals damage equal to its power to target creature you don't control.
If either creature is an illegal target as Hunter's Edge tries to resolve, the creature you control won't deal damage.
If the creature you don't control is an illegal target as Hunter's Edge tries to resolve but the creature you control is a legal target, you just put a +1/+1 counter on the creature you control.
You can't cast Hunter's Edge unless you choose both a creature you control and a creature you don't control as targets.