Minecraft Dungeons

Load the Broken Citadel level on default difficulty and at the start ensure that the basic Enderlings have spawned. Otherwise, return to camp and load the level again. Once the start of the level is full of the basic Enderlings, train them around the various islands until you have a huge group. Bring them to your objective, a firework barrel, and use it to launch yourself into the sky. Position yourself above the large group of Enderlings and attack to perform a dive and hopefully defeat 12+ of them at once.

If you have trouble hitting 12 or more enemies at once solo - like me - then try this method: Load the level Broken Citadel and set the difficulty to default to make it easier for yourself. Check if there are Watchlings (see image above), as it's nearly impossible to get the achievement with other enemy types. Restart the level if there are none.

Lead 12 or more Watchlings close to the second fireworks barrel. There should be more than enough, but do not attack them so enough remain alive to get the achievement. The more enemies are following you, the easier it will become to unlock the achievement. Be especially careful with certain enchantments on your equipment, e.g. enchantments that grant you pets or thorns, which can easily kill the enemies on lower difficulties without you realizing. If necessary, switch equipment so nothing is damaging them.

Use Shock Powder to stun the group of enemies close to the second barrel. Depending on the level of Shock Powder, they are stunned for a couple of seconds and the Watchlings won't be able to move or teleport away. Mine was level 175 which has a 9.9 second stun duration, but one at a lower level works just fine. You actually only need a couple of seconds to launch from the nearby fireworks barrel, line up above the center of the group using the circle on the ground, and perform a dive attack by holding while in the air after launch. If you hit 12 or more enemies in a single dive attack, you'll unlock this achievement. Using this method, I got the achievement the first time trying, so I hope it will help you too if you're struggling.

For those who prefer a video guide (*edit: improved video guide):
If anything is incorrect or unclear, please let me know and I will try to improve this guide.

Best level: broken citadel
Recommended equipment: ghost cloak

This can be obtained near start of the broken citadel level. To unlock this level you must defeat 6 endermen that spawn in certain mission and collect 6 eyes of the ender (they look like small green eyes on the mission select map) and then return these eyes to the portal on the stronghold level. After that you must complete the end wilds level to unlock the broken citadel level.

Start broken citadel and make your way across the floating islands. Make sure that you DO NOT ATTACK the enemies and simply try to get them to follow you until you reach the second firework barrel. Once you have enough near the firework barrel activate it which will send you flying into the air. Fly on top of the twelve enemies and hold the attack button to dive into the ground and hit them unlocking the achievement.

*Edit
Can also be done with a co-op partner who can gather the enemies for you.
Credit to Robby7430

*Edit 2
As more detail was requested and more info has been given to me by M3LANCH0LYM3L0N and Trombonafide.
Depending on which enemies spawns this can be a breeze or almost impossible. The watchlings are the best enemy for this (very close looking to endermen but they are shorter and have purple eyes on their body). The ghost cloak is useful as you can walk through enemies when activated. This is good if you are stuck in the middle of a large group and makes it easier to gather them in a group and escape from a crowd when needed.

If the wrong enemy spawns or you lose too many enemies trying this then you will have to "return to camp" and choose the map again.