Throughout the run, there are 7 hidden "Treasure Chest Guy" hiding in the ground, all of them marked with a red cross on the ground.

The game counts "Treasure Chest Guy" by digging him out, not killing him. This means that you can constantly dig up the first "Treasure Chest Guy" at the beginning of the level, and not kill him. Then you go to another sub-location, go back to the room and dig out the same "Treasure Chest Guy" again, do this 7 times and get an achievement.