Minecraft wiki entry for Spawner_traps, associated text: For this method, you must dig a pit that reaches far under the dungeon. Depending on the height (at least 23 blocks), the mob will die on impact, as long as there aren't any blocks of water at the bottom and the mob has weak enough or no armor. However, this method takes a long time to make and only works for spawners near or above the surface. Alternatively, you can build an elevator for the mobs using alternating water and signs to bring the mobs 23 or more blocks higher, and then drop them 23 or more blocks in order to kill them (or you can do 22 blocks if being used for an XP farm). Keep in mind that skeletons and zombies can randomly spawn with armor, which can be enchanted randomly. This can be remedied by placing a platform of magma blocks at the bottom.