Don't go for big swing - most of your guys will just fly out. If you DO use big swing, either do it with your own player when he has a high power, or if you are good enough with your timing, hit the ball dead-on and right back up the middle. Otherwise, stick with normal and try to hit some stuff out of the infield.
Be patient; take walks, and move your player into wild pitches; hit batsmen are the only way to win the Superstar game. It's a good ploy in the CR as well.
Psychic goggles help you anticipate the oncoming pitch; if its a fastball, it wont move much, so you can use big swing and unload.
Also, items - the Rookie Charm can help your batting immensely.
Finally, play a lot of practice exhibition games in hard modes, without lock-on or anything. That will get you used to the timing of the swing and hitting. Once you can compete with the computer in a hard game, success mode batting gets pretty easy (just ask rsohooo, with his Marvin characters he created.
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