19 Jan 2018
If the creature you control is an illegal target as Hunt the Weak tries to resolve, you won't put a +1/+1 counter on it. If that creature is a legal target but the other creature isn't, you'll still put the counter on the creature you control.
19 Jan 2018
If either target is an illegal target as Hunt the Weak resolves, neither creature will deal or be dealt damage.
19 Jan 2018
You can't cast Hunt the Weak unless you choose both a creature you control and a creature you don't control as targets.