100 Floors: Level 51 is an really simple stage that will probably take you less than a minute to complete. In the stage is a sword above the door and two lights on the left and right. The door is filled with 12 squares where you will be able to turn them black on each tap.
To beat and pass 100 Floors Level 51 you need to tap all of the squares in the middle row to turn them black then tap the second square from the left on the bottom and top row. This creates a sword like image which is also the solution to your problem on 100 Floors Level 51.