A person who engages in sexual activity with an unattractive female on a regular basis. A land monster is an unattractive female that resembles a prehistoric beast that somehow managed to make it on land and find a way to a party.
Frat Guy 1: "Dude, Max hooked up with another fat girl last night. He does that everytime he gets drunk!"
Frat Guy 2: "Good for him. He's definitely a Land Monster Slayer."