(This farm framework does not operate at maximum efficiency after MC 1.13+ by official changes of guardian spawning mechanism. You may search bubble column guardian farm for a better design.)
Credit goes to Sirhc(Youtube) for his original concept.
So if you ask, what have I done? Well I've just recreated the entire main structure which people feel extremely annoying to build especially dealing with waters and hostiles in creative mode, and I've made it a bit more aesthetic. AFAIK there is no such a design post here on this site till now.
This farm is NOT FUNCTIONAL!! (It is the main part). To make this farm functional:
1. You need to FIND AN OCEAN MONUMENT and REMOVE ALL BLOCKS in an 64X64X26 area from the sea level, around and include the monument.
2. ALIGN THE CENTER of my schematic with the center of the monument (the edge of my farm shall be 3 blocks away from the edge of the monument).
3. Make sure 'Without air' and 'With water' are selected in the import selections, and paste it.
4. You need to COMPLETE THE TUBE at the center where the guardians fall, AKA the killing part.
Please do consider my farm no more than a gathering device. I think it shall be easy after all I've mentioned above is done.
After that, you may need to clear some gravel/stone at the bottom layer. The falling height to kill a guardian is 34 blocks I believe, which means you may need something like lava or the cactus trick, to shorten the fall, just for e.g.. Sure you can also use this as a normal mob farm with all sea-lanterns and glass replaced, and it will have a decent spawn rate.
~ Thanks to Sphax BDcraft textures for the screenshots.
~ May all enjoy your games and have fun playing around and farming those crazy laser-shooting bastards!