Boss Mob X (X means Unkillable)
Note: Minify the command to avoid issues :)
This Works!
Blaze Sword
Milk Potion V1
Upside Down CustomShop {NOT a villager!
Pet ENDER DRAGON!!!!!
Wither Storm Boss in ONE COMMAND
HEROBRINE COMMAND
HEROBRINE BOSS BATTLE (See YouTube for details)