Wiktionary
giant-killer
n. Someone or something, especially a sports team, that defeats a larger and superior opponent.
Usage examples of "giant-killer".
I could not avoid thinking that I had fallen in with a greatly traduced people, and I moralized not a little upon the disadvantage of having a bad name, which in this instance had given a tribe of savages, who were as pacific as so many lambkins, the reputation of a confederacy of giant-killers.