For gold you can grind on chapter 3 mission 44 "KING DESTROYER" and for all important items except for the diamond you can grind on chapter 4 mission 78 "Danger, Don't Open!". With the Amazonian I can usually beat mission 44 between 10-13 seconds. In total time it takes about 1 minute and 10 seconds in total from the castle, back to the castle screen again. Cont.