A very fast way to upgrade your character's power is to check out on the screen if you spot the Enhance button. That will appear whenever there is an upgrade which you can do, which will give you more power. I suggest that you keep an eye out for this and whenever it appears on your screen you should tap on it and a new small menu will appear which will tell you which features you can improve (Equip, Buddy, etc). Tap on one of them and it will let you quickly upgrade one of the options. Do this whenever you can make upgrades!