The phrase "be bratty with" refers to behaving in a spoiled, childish, or demanding way towards someone. Some synonyms for this phrase include being petulant, obstinate, difficult, or irritable with someone. Other words that can be used to convey a similar meaning include being temperamental, fussy, sulky, or argumentative with someone. Regardless of the specific word choice, this behavior is generally seen as immature and troublesome, and can strain relationships with others. It's important to communicate assertively and respectfully, without resorting to brattiness or other negative behavior.