Wither!
Note: Minify the command to avoid issues :)
Griefing Items in 1 command 1.8.7 (Read the comments!)
Old steve's head
No Way! Only a block of dirt can do that ?!!!!!
Evil Golems in 1 command
Pet ENDER DRAGON!!!!!
Wither Storm Boss in ONE COMMAND
HEROBRINE COMMAND
HEROBRINE BOSS BATTLE (See YouTube for details)