I'm not talking about を に する but rather に をする

So

NounAにNounBをする

Examples: 恐怖に体を固くする

How one should take that sentence? I've seen some TL mark is "By" but I don't
understand why?
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2023-01-16T00:59:29.360
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2023-01-16T00:59:29.360
55492
question
particles particle-に
Use of the を and に particles together with nouns