Letting the injured shade get away, since she wasn't going in Yelena's direction, Winter had to move to the other side of his current building's roof to get a shot in that won't also go right through Yelena's face, but he wasn't the best shot in the US Army even before HYDRA got their claws in him for nothing. He doesn't need a serum to get a good headshot.

Which he takes right about then, aiming for the shadow woman's temple with his rifle while the light hopefully keeps her solid.