Ok, I have found a simple glitch...
Actually, kaimuro helped find it...
Go to the cave where Pianus spawns... kill Pianus, and then go outside of that map (into the map with the FlyEyes) and kill them...
f the glitch still works, you should be getting WAY more exp from the FlyEyes...
I heard it also works with Snails and other enemies after killing Pianus...