Archievement "Overkill" can be done in Creative Mode.

Playing Minecraft normally in my main Creative World, I realized that you can earn the archievement "Overkill" in this Gamemode (With Cheats active and being the World originally in Creative Mode).

Ways to recreate it:

-Obtain a Diamond Sword with Sharpness V, Fire Aspect II, Looting III, Unbreaking III and Knockback II.

-Find a Horse with 9 or more Hearts.

-Kill it with a Critical Hit (It may took 3 attempts or more, in some cases)

(Obviously you don't need to have the Archievement in order to Recreate It.)
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