This is an update to the 113 Guardian Farm posted. I added lava to start the guardians on fire and a small drop. The guardians now die and leave their drops in the chests, where before they were just flailing around aimlessly... I hear magma doesn't work past 1.11.2 - Got rid of some of the exterior glass, which is really pointless unless you do a lot of other work first (survival probably).
P.S. This has to be built in the place of a Monument. Thus, if you're in creative, just type /locate Monument. Then, /tp x ~ y (coordinates given) and begin hollowing out the monument with sand (survival or creative). Then, hollow out the sand (leaving one layer to server as a wall all the way around), and start building.
You will have to build a special platform below the top in order to get water to flow through the entire tank. Or, you can just use the //wand to //set water between the top and the magma blocks (do not go below the magma blocks with the //set command).