Blue.Dragon.Guide.#2

Medals, Hearts and Elixirs
Somewhere in the game, you will meet Toripo - an explorer that trades rare goods with Medals. Among the goodies that you can buy from him are "Heart upgrades" for your shadows and elixirs that could boost your stats.

My advice is to focus on the elixirs that boost Attack, Defend, Magic Attack and Magic Defend. There is no point investing in HP and MP boosting elixirs as your HP and MP would most likely reach 999 before you even reach level 99. As for the heart upgrades, there are accessories like the Black Belt and Eye Patch that will help you earn double the SP.

What you may discover is that you may have a few characters who are weak in attack or defend even after they reach level 99. You can then use these elixirs to boost their stats.

If you are going ahead with the heart upgrades, make sure you don't use them on a shadow that is at level 98. Doing so will forfeit your ability to unlock the Max One Class achievement.