I was testing stuff with enderdragons in a creative test world. To stress test my laptop AND to try and learn to work with command blocks.
By accident I hooked up a compartor clock to a command block spawning dragons. You guess it. A TON of dragons spawned.
I was able to get to the pause menu but when I placed it on peaceful, they dragons kept living.
Works as intended.
You can use the new @e selector to kill every mob of a specific type.