Minecraft wiki entry for Achievement_guide, associated text: Find a large flat area on the surface. Dig a trench one and a half blocks deep, two blocks wide and several blocks long. Line the trench with light. Just over 50 meters away from the lip of the trench, make a tower 15 blocks high. When night comes, wait for a skeleton to fall into the trench or lure it in. Then from your tower, aim just above the skeleton, about halfway between it and the horizon, and fire from full charge until it dies. With a flame enchantment, you can see where your arrows landed better and can adjust your aim easier, but the skeleton might die of the fire, not giving the achievement.