Minecraft wiki entry for Combat, associated text: This is an even more powerful weapon that can kill a player in Protection IV armor quickly. Simply place down a block of obsidian, place the end crystal on top, and shoot it/hit it. Be aware that in order to deal the maxium damage from the end crystal, your opponent must be in the same y level as the crystal or higher. You can do this by hitting them and then place and destroy the crystal quickly or just wait for them to be on a higher y levels than you. Unfortunately end crystals are expensive to craft, and can only be placed on obsidian or bedrock.