I created a bubble based guardian farm with a lava killing chamber.  The guardians fall off the water stream into the lava chamber and just float there.  If another guardian drops into the killing chamber and touches the first guardian, then it will drop past the first guardian, hit the lava, and then float back up to the first guardian taking fire damage.  I watched this go on for a while, with over 6 guardians floating near the top of the killing chamber.