29 Sept 2017
If the creature you control is an illegal target as Savage Stomp 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.
29 Sept 2017
You can't cast Savage Stomp unless you choose both a creature you control and a creature you don't control as targets.
29 Sept 2017
If either target is an illegal target as Savage Stomp resolves, neither creature will deal or be dealt damage.