To kill Destroyer:
/kill @e[name=Destroyer]
Note: Minify the command to avoid issues :)
Boss Creature Command (1.9)
TheZombieKing
God Pvp Kit
PVP Leggings
Pet ENDER DRAGON!!!!!
Wither Storm Boss in ONE COMMAND
HEROBRINE COMMAND
HEROBRINE BOSS BATTLE (See YouTube for details)