Minecraft wiki entry for Achievement_guide, associated text: 3 elder guardians always spawns in each monument, which generate only in deep ocean biome variants. Its better for you to wear a turtle shell with Respiration III and diamond boots with Depth Strider III. Having potion of water breathing can also help you stay underwater for longer time, or you can also use soul sand to create upward bubble columns, which can give you oxygen plus push entity upward. Its recommended to kill elder guardians using trident enchanted with Impaling V. Strength potion II may also help defeat elder guardian easier. To brew a Potion of Strength