Thanks to him singlehandedly beating four opponents, our high school team won the championship.
Literal
His 4-person beating [possessive-の] effort [with-で] our high-school [possessive-の] team [subject-が] won-championship.
４人抜き ('beating four people in succession') uses ~人抜き, a sports counter for consecutive victories. 活躍 (かつやく, 'effort, great performance') is the standard word for someone's standout contribution. うちの ('our') is the casual possessive. 優勝しました ('won the championship') is the clean payoff. A sports celebration sentence.