giant-killer 
noun
A person or team that defeats a seemingly much more powerful opponent.Rugby's capacity for throwing up cup giant-killers is notoriously rare....
- It was giant-killers to giants in just four days for Livingston, and after six minutes they could well have been slain.
- But although these clubs play in a higher level than the rest, there is absolutely no guarantee that the lower sides will not become giant-killers.
Derivatives
giant-killing
- Amateurs and professionals will be striving to gain this prestigious crown that unites the game's elite and grassroots players with giant-killing on everyone's mind.
- The dream of giant-killing was quickly shattered.
- With the Russian game on December 3, and with no expectations against the others, it could set them up for a spot of giant-killing.