For the sphere implement an on-hit effect that adds impulse at location of the impact to the actor it collided with (other actor).
The above uses the physics engine, it only works on physics enabled objects.
I’m not sure if physics can even feasibly be enabled in the traditional sense for a character blueprint (it’s easy enough for pawns though, and character blueprint is just a pawn with extra features baked in that you might not necessarily need)
The simplest way to do it on a character would be using it’s movement component:
If you want to do high quality knockback you will need to dive a lot deeper and use physical animation:
