Both of these two beings are top of the line fighters. They have vast super strength, Vast Super Speed, and an array of other powers. Superman can shapeshift, and use his Heat Vision and Freeze Breath.
Supergirl also has her Heat Vision and Freeze Breath. Superman has been crime fighting since before the fifties and knows the ropes. While Supergirl hasn’t been fighting as long she still has lots of battle experience and isn’t to be taken lightly.
She loses this match since she’s just not as strong as Superman in the end, but she will be getting wins soon. That’s one thing we can count on. This isn’t an easy match for Superman. It’ll come down to the wire. Superman wins.