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Note: Minify the command to avoid issues :)
More mobs in 1 command (Read The comments!)
Armor Stand killing someone!
More mobs: Burning skeletons 1.8.7 (Read The comments)
More mobs in 2 command (Second one below}
Pet ENDER DRAGON!!!!!
Wither Storm Boss in ONE COMMAND
HEROBRINE COMMAND
HEROBRINE BOSS BATTLE (See YouTube for details)