Wither, iron golems and snow golems and EnderDragons!
Note: Minify the command to avoid issues :)
Witherstorm statue one command
Advance Mob Spawns!
witherstorm one command
EVEN MORE SPAWNS!
Pet ENDER DRAGON!!!!!
Wither Storm Boss in ONE COMMAND
HEROBRINE COMMAND
HEROBRINE BOSS BATTLE (See YouTube for details)