The unfortunate tendency on downhill bunker shots is to blade the ball, either by hitting it before the sand or by bouncing the club off the sand and into the ball (uphill bunker shots are easier). When playing the downhill shot, setup is critical. I align my body with the slope of the sand. My left shoulder is lower than my right shoulder. (If you drew lines through my shoulders and feet, the lines would be parallel.) My weight is on the inside of my left foot. 

I play the ball in the center of my stance, opening my body and the clubface slightly. On the backswing I set my wrists a little more quickly and take the club up abruptly with my arms. Swinging down I make sure to hit behind the ball the same as I would on a level bunker shot. I follow through down the slope, keeping the clubface open and as low as possible. 

What I’ve done is change my address position and the arc of the clubhead to conform to the downslope. I expect the ball to come out lower than usual with more roll. 

QUICK SET — 

I hinge my wrists quickly and swing the club up with my arms. 

DOWN THE SLOPE — 

Through impact, I hold the face open and keep the clubhead low.