To quote Aerostar's Final Fantasy Tactics: Battle Mechanics Guide:
[6.11] enemy levels
Levels of enemy units in FFT can be determined by three different schemes.
FIXED LEVEL: This is used for most enemies in story battles. The enemy's
level is fixed at a certain value and will always be the same, regardless of
the levels of your party members. For instance, Velius is always at Level 31.
VARIABLE RANDOM: This is used for most enemies in random battles. The enemy's
level is randomly chosen based on the highest-levelled member of your party.
This counts all units on your 16-man roster, regardless of whether you
actually bring them into battle. If we call the level of this highest-levelled
enemy level = ((HiLv - [HiLv/8]) .. HiLv)
VARIABLE NONRANDOM: This scheme is used for certain enemies in random
battles and is especially prevalent in the Deep Dungeon. If we use the same
definition of HiLv as above:
enemy level = HiLv + K
where K is some constant (in practice, it ranges from 0 to ~24)
In all cases, the maximum level that can be generated is 99.
In summary, it's entirely dependent upon your highest level character. Even setting up a B team will do you no good. You have really only a few options:
Level down your highest level character(s)
- Go to a level like Fovoham Plains or Zeklaus Desert and go onto the panel that lowers your level by 1.
- Or you can Invite or Train a Mind Flare to use Level Blast on you.
A cool thing to note is that the level-up/down trick is the best way to min-max your characters. By leveling up in a job with good stat growth and then leveling down in a job with poor stat growth, you can keep good stat growth. This is also detailed in Aerostar's guide. And, as it says, don't level-up/down in the same class with good stat growth or you will actually lower your stats.
Level up your low-level character
Carry your low-level characters with high level company. Kill off most of the enemies and destroy the brave of the remaining enemies so that they leave you alone. Additionally, for good measure, lower the attack of your enemies down to nothing and their pitiful attacks won't even matter at that point. Then have your low-level character use skills like chakra on your high-level characters or ranged skills on the remaining enemy for easy XP. Rinse. Repeat. ???. Profit.
You can also use an XP thief tactic, but the above tactic is simpler, safe as you do your thing while the enemy leaves you alone, and pretty much surefire as skills like chakra don't really miss or incur counters.