Gila's Monsters

Once you defeated the boss and got the evil bell, go back to the village and got to the house with the person called Gila lives. Talk to him and he will ask you to go down his basement. Once you do go down make sure you go to the wall where a message will come up. Use your evil bell on the wall and the wall should collapse and monster should start coming out. Behind the crack wall is a large area where monster will keep on spawning. Keep on killing the monsters until the monster stop spawning. Once you did this go into the next area (keep on going straight) and there should be 2 or 3 monsters in this area. Kill them and unlock this achievement.