For my customized version of 
Bad Dreams, I had two more difficult enemies that have much more spread out spawn times. One enemy is a giant Zombunny and the other is a giant Hellephant. The Zombunny spawns every 45 seconds and the Hellephant spawns every 90 seconds. You need to kill them when they first show up in order to not get overwhelmed by all the other enemies. I also changed the music in the game.