well we don't want to get rid of the pronation, we just want it at the right time. when I throw my slider, I like to think about just throwing a fastball, but instead of throwing down the middle of the ball, I get my fingers on top of the outer third of the ball and pull down hard. all I do is slide my fingers over to aid in getting high and outside the ball while I throw. it's simply an off centered fastball I would say haha. to avoid the early pronation you have, maybe having a stiffer wrist while throwing the ball will help you. it's helped me get some sharper break as well.