Children of Morta

This and Stronger Together can both be done with a 2nd controller. Simply connect it and press the start button and select a 2nd character. The achievement can be completed on any level as you are able to go down as many times as needed but I would recommend a level with 2 floors to make the Stronger Together achievement easier. I would recommend planting a 2nd controller on some spike traps and pressing x to revive as this is faster than getting hit by enemies later in the game as you may have 500-600 hp. I would also recommend finishing the dungeon with your main controller with your strongest family member to get the only 2 co-op achievements in this game