Hint:
1) Find an enemy that drops more upgrade stones than normal.
2) Beat the next enemy after him.
3) The enemy that drops more upgrade stones than normal will be in the training fight now.
4) Keep doing the training fight over and over again to rack up upgrade stones!
+ this so other people can know! Thanks and happy hunting!
Enemies don't have a specific drop rate. The drop rate is based on the dungeon part.