Well, I'm no expert, and have some spare time, so here's some things that helped me, not saying I'm right ....
I had flipping lack of compression trouble... I eased up on my excessive head\body movement to the right on the backswing, this seemed to help me set my wrists beter and everything seemed more accurate.
I concentrated on stationary head, and hand, arm (more lift) initiating the pivot\takeaway more (*very* difficult) .. dunno why but this really helped me. I guess I was swaying way too much which led to bad ball striking.
You need to leave the hands behind as you start the downswing, and think hit the ball with pivot body.
My left wrist ain't dead flat (it's flat at the top) but I get tonnes of compression (not saying this is a good thing but still).
Best to practise half shots punch shots with 8 iron. Feel that pivot rotation at impact, leading, and rotatingswivelling the clubhead. Feel like head stays back.. it will be moved.
I don't recommend practising off mats either.
Shanking .. over rotation of club on backswing? (check out Brian's never slice again), or posture\balance? Don't jabb at the ball with ya hands, let the body pivot do it as mentioned above.