Before doing this, make sure you have a grappling hook in your party's inventory. Follow the main story up until you unlock your stronghold, then head back down to the Endless Paths (where you found Maerwald). Fight your way through to the second level. Once here, you'll start in the north-west corner of the map. Pass through the first doorway, then follow the path to the northeast, turning southeast as soon as you can. Through the door at the end of the hall, take the door to the southwest. There'll only be a single exit from this room, leading to a fight against five xaurips (four regular xaurips and one xaurip high priest). After killing them, save your game, as there is no way back, then examine the hole in the middle of the room. From here, you are presented with a choice - jump, climb down, use a grappling hook, or leave. Any of the first three options will work, but both jumping and climbing down will cause issues for your party, assuming you don't plan to reload your save after you reach the bottom (the latter only if you fail an athletics check), hence bringing the grappling hook along. Regardless of your choice, you'll immediately skip floors 3 and 4, immediately unlocking the achievement with minimal work.