To kill Destroyer:
/kill @e[name=Destroyer]
Note: Minify the command to avoid issues :)
KILLER POTION COMMAND
SKELETON BOSS!!!!!!
TheZombieKing
Destroyer (BOSS)
Pet ENDER DRAGON!!!!!
Wither Storm Boss in ONE COMMAND
HEROBRINE COMMAND
HEROBRINE BOSS BATTLE (See YouTube for details)