Enter The Gungeon

To unlock this, you need to push a table into a pit/void. You can find tables in some of the rooms, but since the rooms are random, you may have to look around for a bit. Pits are also random, so finding both of them in close proximity might be challenging.

Once you have a room with a table, and a room with a pit, tip the table over and push it into one of the pits. All tables should be able to fit through doorways, so you can move them between rooms if necessary. Once you push the table into the pit the achievement unlocks.