This is a very easy exploit. Just acquire a shield that also counts as a weapon, such as the Gouf Custom's Gatling Shield or the Strike Rouge's Combination Shield and sell it for GP. As long as you have not updated your game, you can sell it an infinite number of times. Note that the level of the part has no effect on the price, only the rarity, so to maximize the rate, Pair Merge the shield with a shield or ranged weapon of the next highest rarity whenever you can. Then, simply button mash until you have the money you need.