「もし十香が抱いたような感想を、皆が感じてくれていたなら御の字ね。みんなの戦意を高揚させるのも司令官の仕事だから。ーーでも、ただ熱狂するだけじゃあ駄目。頭はクールに、ハートは苛烈に、が理想ね」
頭はクールに、ハートは苛烈に sounds like a proverbial expression. Nevertheless, I’m curious about the grammatical function of the bold に. Could you please explain that?
「頭はクールに、ハートは苛烈に」 is not an existing idiom nor a cliché, but it does sound like a catchy slogan because the corresponding verb is omitted. In this case, you may think something like しよう or 保とう is omitted after each に. (This に is technically a particle that turns na-adjectives into adverbs (i.e., "-ly"), but here it has a resultative function.)
Here are similar examples. Words in parentheses are omitted verbs.
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