Dark Knight is one of the most difficult Dresspheres to obtain. When you enter Bevelle Underground (If you sided with Youth League you will go here automatically at the end of Chapter 2 To look for Vegnagun), Continue going down and along the paths until you will come to three lifts. Take the one on the right down first. Then step climb up the two small blocks on this level. After that, walk all the way down on the screen. This will take you back to where the three lifts were however the one on the right is gone now.
Then take the center lift up and climb up the block there. This will raise some blocks on a different screen. Then take the lift back down and then take the left lift down this time. Now go to the right side of the screen and take the right lift back up. You will be back where the three lifts were but this time the left lift will be missing. Take the center lift up again and climb up the block to raise some other blocks on a different screen again. Now go back down and then take the right lift back down to bottom. Now go up to the bridge and get on the last lift. This will take you up to the next screen where you were raising the blocks before. Jump across them to grab the Dark Knight Dressphere from the treasure chest.