、。
There's an expression 'once-in-a-lifetime grand occasion,' and this is exactly that.
Literal
Once-in-a-lifetime [genitive-の] grand stage [called-という] expression [subject-が] exist [but-が], this [topic-は] exactly that is.
Built around two set expressions. 一世一代 (いっせいちだい) is a 四字熟語 literally 'one generation, one era,' meaning 'once in a lifetime' — typically used for a moment so significant it only comes once in someone's life. 晴れの舞台 ('stage of glory' / 'grand stage') is a fixed metaphor for a crowning public moment — a debut, a final match, a wedding. Together they form the kind of elevated phrase that novels and speeches reach for. ~という言い回し frames the expression itself as the topic ('there's an expression that goes…'), and まさに ('exactly,' 'precisely') collapses the cited expression onto the present situation. The sentence ends in である, the formal/literary copula, keeping the register elevated.