When thinking of antonyms for the phrase "more mild-tempered," one could consider words that denote a more aggressive or volatile nature. These might include terms like irritable, aggressive, hot-headed, or volatile. Other possible antonyms might include words that describe a more passive or non-confrontational attitude, such as meek, submissive, or timid. Conversely, words like confident, assertive, or strong-willed might be seen as antonyms for mild-tempered, as they suggest a more forceful personality. Ultimately, the antonyms for a phrase like "more mild-tempered" will depend on the specific context in which the term is being used and the qualities being contrasted.