Punch above one's weight
Competing against someone who you are no match for.
The different classes of contestants in boxing matches are distinguish by the weight of the competing boxers - heavyweight, middleweight, lightweight, flyweight etc. The sport is regulated so that only boxers of the same weight fight each other. Someone from a lighter weight wouldn't be expected to have much chance if 'punching above his weight' against a heavier fighter.
The term is often used figuratively in situations where someone finds themselves competing outside their usual class; for example, the Irish comedian Graham Norton described that, since becoming well-known, he was able to attract better-looking partners than previously and that he was 'punching above my weight' when it comes to relationships.