That's why there are so few Pros.  I am working on the same thing.  I have found if I visualize a spot on the ground beyond the ball  and  bring the club head thru with a smooth tempo I can do it.  I seem to shorten my arms thru contact so when I do it right it seems like it takes a long time,

The best way I use to create a downward strike, especially for 7 iron through LW is the stack and tilt style of weight shift.  Keeping more weight an your lead side, and feeling like there's no weight shift during the backing, naturally creates a steeper angle of attack. 

I don't like the phrases "hit down on the ball" and "trap the ball between the clubface and the ground"

Because they present a picture of actually hitting down on the ball, that doesn't happen, neither does trapping the ball between the clubface and the ground.

Your swing moves down and away from your body, at some point the clubhead reaches its lowest point, you want to hit the ball before the clubhead reaches the lowest point. The clubhead is moving downward at impact, the face still strikes the ball below the equator of the ball, not above the equator of the ball.

At address you should have your hands a little ahead of the clubhead, at impact you want your hands in front of the clubhead, and you want your weight over your front foot(or at least moving forward). These 2 things, hands leading and weight moving forward lead to good solid impact.

Try putting ball in middle of your stance and digging instead of sweeping.