To unlock this achievement, you must have the upgraded necrorod, obliterator, and access to Lighthouses unlocked.
Then, using an obliterator, you need to dig an area 24 blocks wide and 12 blocks long.
After digging out the space, the first two rows starting from the top you need to fill with lightouses.
Then add below them 3 columns (of lighthouses) on the right and left side. (You can check here how it should look
https://prnt.sc/lE2MP90CCY59)
Then you have to add 2x2 island in the middle and put on it the spike traps.
Now all you need to do is spawn a lot of skeletons using the necrorod and then sell their heads and bones on the market. (To make it go faster, you can use the liquid luck and greedy potions and when selling, hunt for the "merchant" shrine upgrade, which will allow you to sell at double the price).
After a few minutes of spawning skeletons and selling items, the achievement should appear (it took me about 20 minutes).