With this sentence
What better way to celebrate the peace than with a fight?
Is "what better way to" an informal phrase? Would it be better to write
What is better than fighting as a way to celebrate the peace?
Learn English – What better way
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Best Answer
This phrasing highlights the irony in the statement (a fight celebrates peace) by juxtaposing the words in close proximity.
"What better way" is an English idiom which is rhetorical. The question doesn't literally ask for a better way, it posits that there is no better way. I don't feel the idiom is particularly informal, I don't think you should worry about using it in formal writing.
This phrasing sounds needlessly clunky, and comes across sounding like you are seriously asking for a better means of celebrating the peace, since fighting is clearly not sufficient.