While playing in a survival world (must be survival), wait until nightfall and enemy mobs will begin spawning. Attack, ideally with a sword, and kill one of the many zombies, skeletons, spiders, and creepers, unlocking the achievement.
I've contributed to making a world/walkthrough earning you all achievements really easy. You can check out the walkthrough on the following link. https://www.trueachievements.com/gam.../walkthrough/1
The achievements are grouped per update, although if you are somewhat familiar with the game, just hop into the world and do the achievements without the help of a text guide. Please let me know if you need any help.