This achievement is for winning a ranked match with a handicapped character. Handicap characters are the ones with a golden skin that start every round frozen and have a much smaller boost meter than usual.
You unlock your first golden skinned character at level 9, which is a skin for the Speedrunner character.
You might want to practice on unfair bots and try to get this achievement against entry league opponents. (or alternatively get good in ranked games and keep searching until you find an entry league person, but it may be harder to find them when you're higher league) Since they are still learning the basics of the game, you can likely beat them even with a handicap if you practice enough.