Put a +1/+1 counter on target creature you control. Then that creature deals damage equal to its power to target creature an opponent controls.
As Felling Blow tries to resolve, if either creature is an illegal target, the creature you control won't deal damage. If the creature you control is a legal target but the other creature isn't, your creature will still get a +1/+1 counter.
You can't cast Felling Blow unless you choose a creature you control and a creature an opponent controls as targets.