Cat Quest III

The 8-bit dungeon can be found in the NW corner of the map. You'll need to sail your boat all the way up to the gate and shoot it with your cannons to access the dungeon. This is a high-level dungeon, which I did around Level 52 with no problems. You'll need to travel up the floors, defeat the boss, and collect all three treasures to unlock the achievement. The first treasure is directly after the boss, the second is behind a gate that unlocks after defeating the boss in one of the middle floors, and the final chest will spawn once you return to the room with the painting.