Hi, I'm drawing a manga and there's this action scene where a shirtless guy gets punched in the stomach. Can someone tell me any tips on how to draw this?

hmm.. there's a 3 step panel process.

Beginning of action-
Contact-
End result-

I say draw it from an angle you feel comfortable with... don't forget the expressions of pain on one, anger on the other, which is generally the cliche concept of someone punching someone else in the stomach.

When you get punched in the stomach without knowing it, your diaphram tightens and causes you to lose breath and fold over.

If a trained person sees the punch coming there are ways to take the punch w/out much damage or the other person effecting them.

It's usually purely situational.

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That shape. Like Ego said, it folds over. Then again, that depends on the strength of the punch and the readiness, strength, and skill of the guy being punched (and the guy punching, I guess).

the guy getting punched is so unprepared that the puncher's fist is engulfed by the guy's stomach. plus, the punchers punch is super powerful. what i need to know is how to draw his musceles and skin as he follds. can someone post an example?

Try watching a martial arts based anime like Histories Strongest Disciple: Kenichi. You can pause the anime and use it for reference. Or find an action filed manga.