Most of the special characters work best at the unique job class that they are given upon joining your party. Mustadio works well against undead creatures with Seal Evil, surprisingly.

Quote Originally Posted by VeloZer0 View Post
Tweak for speed
Equip all the +speed crap on him you can regardless of HP and then equip him with Throw Item. Items get outclassed by White Magic at the middle of the game, but I find that by the end game the instant cast of Items makes up ground as all the characters get faster. That and the rest of your uber party probably don't have any healing abilities equipped

Mustadio has a little bit of innate synergy with the Chemist class as both Engineer and Chemist can freely equip guns without having to equip a support ability. I used Mustadio as a Chemist for awhile until he mastered the class as it allowed him to keep his role in battle without changing anything significantly.