Target creature you control gets +2/+2 until end of turn if its power is 2. Then it fights target creature you don't control. (Each deals damage equal to its power to the other.)
(14/06/2019)
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.
(14/06/2019)
You can’t cast Savage Swipe unless you choose both a creature you control and a creature you don’t control as targets.
(14/06/2019)
If either target is an illegal target as Savage Swipe tries to resolve, neither creature will deal or be dealt damage.
(14/06/2019)
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.