In order to maximize a Yo-kai's stats during Official Battles, two mechanics must be utilized: the Springdale Sports Club, which can give up to +25 to a single stat, and Attitudes, which can give up to +26 to a single stat. While slowly Leveling a Yo-kai to Level 60 was previously thought to be the fastest way to get the maximum Attitude stat increase, there is a much quicker way. This method also raises a Yo-kai's Seriousness to its maximum, eliminating the need to use A Serious Life.
The reason this works is due to how Attitude stat gains work. Every Yo-kai has a hidden Personality value. Completing a PvE battle gives that Yo-kai one Personality Point. The full benefit of Attitudes is reached at 300 Personality Points, resulting in Attitude point gain being very slow. At least, it would if Attitude points could not also be gained by using Exporbs. Using Exporbs on a Yo-kai gives the following amounts of Attitude points:
Raising Personality also Levels the Yo-kai's Seriousness. This can save a large sum of money by eliminating the need to use A Serious Life.
Credit for Personality information goes to the wonderful Togenyan. Thank you for your contributions to the Yo-kai Watch community.