Use Training Items and Locations Wisely
Save your good Training items and locations and use them for your SR characters up. Training Items are hard to come by so spending the low quality training items on R's and not using them on Normals.
Leveling up the Normal Fodders
To level up your fodders, have a strong team composed of at least 4 powerful characters then add two fodders as support. Bring them to a story mode's boss map and keep clearing it over and over again until they hit LV20.
Z-Awakened Fodders
The best way to get the most EXP when Training a character is to feed him/her with a Z-awakened Normal. Normal characters can easily max out at LV20 and they only require two Types of medals, three pieces of each. A max level normal gives 3,180 EXP compared to a Z-awakened normal (Turned R) that gives 44,670 EXP. If the Z-awakened fodder is fed to a character with the same color, that character gets a 50% EXP bonus. Take note that even if you max out the Z-awakened Normal to LV40, the EXP amount it will give won't be double the amount of when it was Z-awakened.