4 Heroes of Light - Broken "Merchant of Death"
Generally, I share the view that FF: T4HoL has an exceedingly balanced system of classes and there are about two jobs that I feel to be useless for combat purposes: Wayfarer and Monk. The former is only good for escaping from battles and the latter is too useless for being so gimmicky.
Sure, some classes are more useful than others, but there aren't any besides the two mentioned that you would look at and say, "I'd never even think of using this class."
But there's one class that's all too notoriously broken... Yes, Merchant.
You get the class fairly early in the game and unlock Finder in addition to the Keeper ability to gem-farm easily. However, it's the last two Merchant skills that can seriously break the game. Ransom and Money Talks.
Ransom, after being used once, will null any damage dealt to the Ransom user at the expense of your Gil. I think you lose Gil 10x times the damage amount. If you really want to know, count the change in this recording based on how much my Merchant blocks. The thing is, if you farm a lot of Gil and equip a Ribbon so that certain accidents don't happen, you can say you're immortal.
Then there's Money Talks, this one's just as broken as Ransom but in the offence department. Money Talks deals damage equal to 1% of your current Gil, wasting up to 1K Gil per use, iirc. Since Gil-farming methods become available so soon, you can one-shot every enemy in the game assuming you farm for long enough, including all the final and bonus bosses.
Here I show just how overpowered the Merchant class is by fighting the Ogre Bear. He does crazy damage and attacks thrice per turn but if we survive to use Ransom, we're invincible. Money Talks kills him in one shot with the amount of Gil I have, too.
So yeah, if you don't want to spoil your playthrough, don't use these abilities.
Playing a low level challenge right now, I made a point to gain a couple additional levels in the bonus dungeons to get special equipment instead of using these skills to beat the game at 1 or 2 levels earlier... Let's see how that goes.