Note: Minify the command to avoid issues :)
Diamond Man (1.9.1)
Dark Opal Demon (Boss) 1.11.2
Northroniss,Nightmare of the skeletons (REALY POWERFUL!)
More mobs: Burning skeletons 1.8.7 (Read The comments)
Pet ENDER DRAGON!!!!!
Wither Storm Boss in ONE COMMAND
HEROBRINE COMMAND
HEROBRINE BOSS BATTLE (See YouTube for details)