You can’t cast Savage Punch unless you target both a creature you control and a creature you don’t control.

If either target is an illegal target when Savage Punch tries to resolve, neither creature will deal or be dealt damage.

Assuming the ferocious ability applies, if just the creature you control is an illegal target when Savage Punch tries to resolve, it won’t get +2/+2 until end of turn. If the creature you control is a legal target but the creature you don’t control isn’t, the creature you control will get +2/+2 until end of turn.

Some ferocious abilities that appear on instants and sorceries use the word “instead.” These spells have an upgraded effect if you control a creature with power 4 or greater as they resolve. For these, you only get the upgraded effect, not both effects.

Ferocious abilities of instants and sorceries that don’t use the word “instead” will provide an additional effect if you control a creature with power 4 or greater as they resolve.