The Last Kids on Earth: The Graphic Novel(original) Max Brallier

Ever since the monster apocalypse hit town, average thirteen-year-old Jack Sullivan has been living in his tree house, which he's armed to the teeth with catapults and a moat, not to mention video games and an endless supply of Oreos and Mountain Dew scavenged from abandoned stores. But Jack alone is no match for the hordes of Zombies and Winged Wretches and Vine Thingies, and especially not for the eerily intelligent monster known only as Blarg.