Wither!
Note: Minify the command to avoid issues :)
Better EnderDragon (4 commands)
Armor Stand killing someone!
More mobs in 2 command (Second one below}
FIREBALL THAT CAN BREAK ONLY OBSIDIAN !!
Pet ENDER DRAGON!!!!!
Wither Storm Boss in ONE COMMAND
HEROBRINE COMMAND
HEROBRINE BOSS BATTLE (See YouTube for details)