TRIALS of MANA

This achievement cannot be missed. You must have Kevin in your party. Once the game story has completed, go back to the title screen and load your completed save. This reverts to before the last boss fight (regardless of ending) and a new quest appears to collect spheres, that unlock a fourth class change.

Complete the quest for the Sphere of Instinct. Once you instigate the fourth class change, the achievement will unlock.