You could simply apply an impulse to the player when they are hit.
Create a vector in the direction of the impact ( PlayerPos - FireballPos ), Normalize it, and then scale that vector by some scalar for power (you’ll likely need to play with this value a bit, maybe put it in your options or somewhere so you can easily tweak it as needed ).
Finally just call SetPhysicsLinearVelocity with your new vector (make sure you set the Add To Current option to preserve their current velocity) and that should be it.