For diamond farming there is a level called "Close-Quarters" or something similar in chapter 5 I believe. There is a dragon boss with two birds & nests immediately next to the castle & a minotaur, purple goblin, & white demon. Also two turrets on the upper-left & lower-right of the map. You can boomwhirl the birds, boomerang boss, regular attack/boomerang combo enemies & dash the turrets. 12-15sec