Got Mystic Knight and... beastmaster. This isn't looking too well.
Mystic Knight is as good as if not better than a black mage. You lose rods (which are mainly used for trivializing World 1 and the occasional lategame Wonder Wand shenanigans) and MT spells (mostly useful for random encounters and the occasional boss gimmick), but your single target damage skyrockets (and inflicts instant death when it hits a weakness on anything that isn't marked heavy), you've got much more fun armor to play around with, and most of all,
status spellblades ignore magic evade. The vast majority of bosses, including two parts of Neo-Exdeath, have some easily cheesed status weakness.
Beastmaster may not be that useful in random encounters, but if you know what you're doing he's got access to most effects in the game, even if playing Natural severely nerfs his Pokemon cannon (and means that you can't trivialize Omega with !Calm). !Control should also make leveling a non-issue once you hit World 2 (use it on Shield Dragons in the sealed castle for 10k exp)
Rebecca's next job will either be Samurai or Dragoon. Dancer is in the 750 category. Beastmaster makes everything go easy, up until the cleft. Then it loses a bit, but its not BAD. Mystic Knight is AMAZING. Not so much call for Pysch, and with the party there is minimal chance of being able to do Fork Tower for Flare and Holy, but status effects on command and drain for spamming attack and not dying, its not too bad. Actually, I THINK there may be a way to do Fork Tower, if you can figure out which creature casts reflect or big guard or something when released.
Protip: Go to the pirate cave and capture goblins for Flare.
As long as you're not playing the iOS version, you can equip the left side party with reflect rings and absolutely nothing will be able to touch them. For Omniscient, you can just let the game run while he impotently empties his entire stock of MP onto your party, leaving him unable to cast reset while you wail on his nerd ass.
IIRC !Release is based on the level of the captured monster rather than the caster, so even though goblins cast Flare it's only going to deal ~900 ST damage.