Enter Rafflesia School (after seeing the king) and go to the first course. There are 4 Malins in the grass. Head north, go down the steps, and hit the right switch to reveal 6 Malins. Then leave (exit the school). Re-enter and repeat as desired. (10 coins a visit x # of visits = $$$)
Eliminate Enemies
There are some areas where, once you beat a certain boss, all of the enemies are eliminated from that area. Here is a partial list of those bosses and areas.
Boss Area
MotherMonster Dahlia Valley
Puppetmaster Iris
Leviathin Root Temple
Baron Anemone Beach