I know the pattern「AがBに劣る」means “A is inferior to B”. But in this context it doesn’t seem to be the case. So what would be the function of に here?
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Here is the relevant sense, as defined on 精選版 日本国語大辞典’s entry for に (translation attempt mine), found on コトバンク:
⑭ 成否、巧拙、好悪などを問題にする対象を表わす。 ("marks an object with respect to which one's success/failure, skilledness/unskilledness, like/dislike is being discussed")
Or less relevantly (I think):
⑮ ある属性や能力を持っている対象を表わす。 ("marks an object with respect to which one has a particular attribute or ability")
More practically speaking, it is mostly used to indicate a quality you have a lot or little of, or something you are good or bad at, similar to the English preposition "in", as in these examples:
・分別に欠けている "lacking in discretion"
・変化に乏しい "deficient in variation"
・ユーモアに富む "rich in humor"
・技芸に長けている "excellent in arts"
・耐久性に優れている "superior in durability"
So our problem at hand 一撃の威力に劣る translates (literally) to "is inferior in (terms of) the force of one strike".