Dig out where the red is a bit, so that the Mushroom Biome is juuuust barely off-screen, you may need to add some mud and mushroom grass up here as well to make sure you're still in a "Mushroom Biome" to the game, then simply put wooden platforms there (The item, not Wood Blocks) and stand on them, then you should start seeing some enemies.
The second thing I guess I'd do, but won't work as well, if for some reason the first one didn't... Stand over by the Green, and put some platforms right about where the green line is, and move back and forth, you will likely have to kill enemies that aren't even mushroom enemies over by the Purple Underground "Moss" and your Hellevator.
I guess I've had a Truffle Worm "Farm" that worked well, and it had water, however, this makes it more likely for things like Anglers, Goldfish, and Jellyfish to spawn, and the Slimes will all somehow find it.. and while you're there killing all the enemies... bats will find it too.