The assassin couldn't have evaded the punch, Reeva was observing her movement and used a gap in his clash to deliver a strike. It was rather easy for him since the bulk of the job Was done by the knight fighting with her.
He just needs to act when the assassin is caught in the sword clash and boom she can't dodge his attack.
His punch connected with her back, but once again, there was no sound. It was unnerving, as if something fundamental to life was missing.
That doesn't matter to him right now, the moment his punch connects he releases the yellow fog into the body, hoping to incapacitate her. But instead of laughing like the other people when they got yellow fog inside them. Reeva could see that she was Ok.
At that moment Reeva also relied on something major, her power had something to do with the void. because there's no way a normal human could take his mystic force in and be fine. or any mystic for that matter.