Target creature you control gets +2/+2 until end of turn. It fights target creature you don't control. (Each deals damage equal to its power to the other.)
If either target is an illegal target as Savage Smash resolves, neither creature will deal or be dealt damage.
If the creature you control is an illegal target as Savage Smash tries to resolve, it won’t get +2/+2. If that creature is a legal target but the other creature isn’t, the creature you control still gets +2/+2.
You can’t cast Savage Smash unless you choose both a creature you control and a creature you don’t control as targets.