When you are in the area where you are just about see the intermission sequence with Luis dying, there will be a door that lets you know that you will need another person for it. After you get Ashley back, return to this door. Give Ashley a piggyback and she will open the door. Inside will be a lot of treasure, and the powerful Broken Butterfly. You do not have to buy it from the merchant or sell it for money. Additionally, the strongest Magnum is not the Killer 7. When you tune up and buy the exclusive option of the Broken Butterfly its power will increase to 50.0, far stronger than the Killer 7's 35.0. You can kill a Boss in four shots.