Minecraft Dungeons

This will happen before you complete the story or be close to unlocking. Just check your achievement progression bar if it hasn’t unlocked and replay a dungeon/level that has a lot of enemy’s and grind that dungeon repeatedly. Edit forgot to say that after you beat the game you can choose a level with a lot of enemy and play it on easy until you have enough for the achievement to pop.

this is going to be unlocked through natural progression of the missions, if i could give any advice on how to speed this process up it would be that you tackle everywhere on your map which you can access by hitting and seeing your map, it does show you when you've entered the area just go to all the outskirts and avoid the main objective until all mobs are defeated! complete this on longer missions like Redstone Mines and a secret mission that you can find in the mission Creeper Woods i do suggest looking at a guide to find that bonus mission as its very helpful for leveling up and mob farming! it's called Creepy Crypts, run through those a few times and you should get it in no time!