Depends what your issues are, but when I first started showdowns, I'd go into custom practice on against the same pitcher I was going to face, at the same time of day, same stadium, same difficulty. Then I'd just sit there and let him throw some pitches. It got me used to the pitcher's delivery and the movement of the different pitches and the speed. I'd let him throw a pretty good # of pitches and then I'd spend a few at bats actually swinging.
Another tip and you can't base this off custom practice because showdown sliders seem way different, but I have found over and over again that each pitcher has patterns/tendencies, even as the CPU. When you're in the showdown, there are always certain pitches that will eventually be thrown in certain spots (approximately). I would figure those out, pick out the most hittable ones and wait for them.
Example: Miller/Ottavino throw a lot of sliders to oppo-handed batters that end up just outside of the middle of the plate and hang around belt high. That is all I wait for with them unless I have 2 strikes.
I can't remember who the AL Central first boss is, but look for a similar sort of pattern with their pitches and look for something to hit.
I also agree with what Josh said about avoiding double plays. If that is your biggest problem, don't be afraid to bunt batters over. Seems like a waste but 1 out and runner on second is a lot better than 2 outs and no one on if that's a common problem for you.
