You can’t cast Savage Swipe unless you choose both a creature you control and a creature you don’t control as targets.

The target creature you control gets +2/+2 only if its power is exactly 2 as Savage Swipe resolves. It’ll still fight if it doesn’t get +2/+2.

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

If the creature you control is an illegal target as Savage Swipe tries to resolve, no creature gets +2/+2. If that creature is a legal target but the creature you don’t control isn’t, it still gets +2/+2 until end of turn if its power is 2.