Minecraft wiki entry for Advancement_guide, associated text: To complete this advancement, you need to kill a skeleton with a projectile that is shot from at least 50 blocks away horizontally. The best way to do this is to first trap a skeleton into a 2x1x1 block area, using fences, so that you don't have to deal with a moving target. Also, make sure that it is nighttime so that the skeleton will not burn in the daytime. Alternatively, if the skeleton has a helmet on, it will also not burn in the daylight. To ensure that the skeleton doesn't despawn, put a name tag on it.