The Karateka (Black Belt in this version) was made an Earth Crystal job.

Your stats as a highly leveled job only increase as that job, if I recall correctly. This is why it's important to actually level up your job levels in this version, whereas I don't remember it being that important in the original other than staying as either a melee or magic class the entire game.

Your lack of healer is probably what's killing you. You should always have a healer on your team without exceptions for the entire game. Either a White Mage or a Bard with the appropriate harp should suffice (unfortunately I don't remember what all the effects and what names of the harps are at the moment). You can also use your allies' critical HP status to your advantage by having a Knight with two shields use the Defend option, which should nullify it's physical attacks (a popular strategy amongst crazy LLG people). I think I remember him having an Earthquake attack or something of that sort though.

Your Thief is outdamaging your Dark Knights because, well, he's a level 80 Thief and they're level 20 Dark Knights. Job Levels really do make a difference in this version. And the Thief besides (and most of the other jobs) have definitely been powered up with the rebalancing of the classes.

Anyway, I'd recommend changing whoever was a White Mage before (looks like Refia) and using them to heal your party.