Target creature you control gets +2/+2 until end of turn. It fights target creature an opponent controls. (Each deals damage equal to its power to the other.)
If the creature an opponent controls is an illegal target as Wild Instincts tries to resolve, but the creature you control is still a legal target, the creature you control will get +2/+2, but the creatures won’t fight. Neither creature will deal or be dealt damage during the resolution of Wild Instincts.
You must target both a creature you control and a creature an opponent controls to cast Wild Instincts.