Minecraft wiki entry for Guardian_farming, associated text: To force guardians to spawn within the farm itself, it may be helpful to remove all other water from the spawning zone. Keep in mind that this is a tedious job and may take many resources and a lot of time to accomplish, as well as guardians being present throughout the process, making this job difficult without good armor. Invisibility potions work against the guardians if the user isn't wearing any armor, but guardians will notice them if they get very close.