The push-back push-up forces your shoulders to work hard to drive your body back using a natural arc and push angle.
Just drop down, explode straight back, and return to push-up position. To take it up a notch, put your feet up on a bench. Your abs will be begging for mercy too.
The flye is tough at the bottom of the movement but easier at the top. To match the strength curve and build more muscle, add manual resistance at the top.