。
She won the contest.
Literal
She [topic-は] that contest [at-で] won.
優勝する ('to take first place, win the championship') is the dedicated verb for winning a competition outright — claiming the top spot, not just one round. Compare 勝つ (general 'win', a single match), 一位になる ('come in first'), 優勝する ('win the whole thing'). The kanji 優 ('superior, gentle, gracious') runs through 優れる ('excel'), 優しい ('kind'), 優秀 ('excellent') — a versatile root for excellence. The で marks the venue of the winning, parallel to the で of 一等賞を獲得する.