Infinite Guitars

You'll have to talk to Hana at two different doors and pay 10,000 scrap each. Each time, there is an adjacent area to grind scrap marked by a blue X on the map. I went to the second area and attacked the same pulsing robot near the NE part of that map. You gain about 400 scrap per battle, so it should take 25 battles at most, if you walked into that area with 0 scrap, which isn't likely. I think I got it in 15 battles, and unlocked two other achievements while grinding, as there are multiple cumulative ones to get in this game.