Spoiler train fight.
Theo (purple) and Venus (red) can be mini, slowed and poisoned.
Locke (blue) and Theo can be berserked, but that isn't very smart unless you blind them with ink (venus can't be berserked or blinded).
Theo can be paralysed (dazers).
All of 'em can be death sentenced.
Locke and Theo counters magical attacks, while Venus counters physical attacks. They counter these with magic.
Theo and Venus also have a general counter, meaning they counter anything with an attack (not magic). That's why it may seem as they get way more turns than you. For instance if you attack Theo with magic, he'll first counter the attacker with thundaga then counter attack a random target. And if it's his turn he'll throw in an additional attack. Then imagine you have a character on standby, ready to magically attack Theo
Here is a decent strategy. Make sure you're resistant to ice, everyone should be able to heal and have one character with added effect + bio and added effect + mini on the weapon. You should be long range and hight attack power.
Start the fight by maigcally mini Venus (red) and keep Theo paralysed with dazers. Cast bio on venus until she's poisoned. Then attack Theo (purple) physically. Since you have added effect + bio + mini he should eventually be mini and poisoned.
If you manage to do this the fight is pretty much over. But you don't have much time.
1st kill Venus with magic (not fire based), then theo with physicall attacks and finally locke with physical attacks.
/spoiler