System's Twilight Hint Guide:

Keep an Ear Out

How do I open the gate?

You'll need Port Key B, which is somewhere in the Net.

What's going on with the puzzle?

In addition to your previous step getting removed, all the notes in the lowest row are removed. In other words, since there are 8 rows, you must win in exactly 8 steps, and you cannot be on the n'th row after the n'th step.

Show me the solution.