I know figures like Hercules and Theseus killed monsters but were there any characters or groups who were known as a monster-slayer? Not just Greek mythology but any you can think of please.
There is actually a hero literally named "Monster Slayer" in Navajo mythology! He and his twin brother "Born of Water" are famous for being the ones who killed most of the primordial monsters. Their story is told in Diné Bahaneʼ, the creation narrative of the Navajo.
Also the legendary Sumerian King Gilgamesh slays a couple monsters in the Epic of Gilgamesh.