ひとつだけ小さいなパンチので…

[Humble amateur]

It might be too early for me considering my current ability to speak Japanese, but I really wanted to give it a try.

Only one little punch and she's already at her limit?
ひとつだけ小さいなパンチので、彼女はもう自分の限界である？

[becki_kanou]

Are you talking about punch, like a punch to the face, or punch like an alchoholic drink?
Also, did you run that through a machine-translator? It sounds suspiciously babelfishy. (Forgive me if you didn't.)

[Humble amateur]

Are you talking about punch, like a punch to the face, or punch like an alchoholic drink?
Also, did you run that through a machine-translator? It sounds suspiciously babelfishy. (Forgive me if you didn't.)

A punch to the face with a bare fist. No, I don't run anything through translator machines. Sorry if it's that bad...

[kurisuto]

（小さい）パンチ一発だけで彼女はもう限界だ？

発 is the right counter for punches. Now, I put "chiisai" (by the way, it's either "chiisai" without -na or "chiisana" without "i") in parentheses because it doesn't sound natural to me in this instance, but it may be fine.　自分 is not necessary, and you can omit "no" in "node" (anyways it would have been "na node").

[Hyperworm]

Maybe it'd be better with か rather than だ?
Otherwise I think that's a good wording.

[Yudan Taiteki]

I think either だ or か works depending on exactly what the connotation is (assuming the だ has a question intonation).