as cool as a cucumber

落ち着き払っている／冷静な／ゆったりしている／すっきりしている

Ex

• You should just stay as cool as a cucumber, even in such situations.

Reference source

• https://native-phrase-blog.com/as-cool-as-a-cucumber%EF%BC%9A%E8%90%BD%E3%81%A1%E7%9D%80%E3%81%8D%E6%89%95%E3%81%A3%E3%81%A6%E3%81%84%E3%82%8B/