וְהֵמַתָּ֛ה אֶת־הָעָ֥ם הַזֶּ֖ה כְּאִ֣ישׁ אֶחָ֑ד וְאָֽמְרוּ֙ הַגּוֹיִ֔ם אֲשֶׁר־שָׁמְע֥וּ אֶֽת־שִׁמְעֲךָ֖ לֵאמֹֽר׃
If then You slay this people all at once, the nations who have heard Your fame will say,…
(The above rendering comes from the RJPS translation, an adaptation of the NJPS translation. Before accounting for this rendering, I will analyze the plain sense of the אִישׁ term, by employing a situation-oriented construal as outlined in “Notes on Gender in Translation,” pp. 11–16.)
Prototypically, אִישׁ is used when sketching a situation schematically. The expression כְּאִישׁ אֶחָד encapsulates such a sketch. It evokes a stereotypical situation, which is used to describe the manner of action under discussion. Its meaning generalizes as follows: “as if there were a single defining participant [in the situation under discussion].”
For analysis of the Hebrew idiom and various English renderings, see my comment at Judg 6:16.