Now listen, folks, leveling up in the WOB is not a very good idea at all, never get past LV35.
You wanna spend every level on boosting your stats in the end of the game, anyway.
Alternatively equip proper Espers while in the WOB, in order to boost stats at the same time, but that's too complicated and stuff, with all those characters.

A good way to boost Sabin's strength to the maximum is to always have him use Ifrit in the beginning,
then change to Bismark.
DON'T CHANGE THAT ESPER!
Let him develop his Vigor to 128, and he'll be the badass!
Then develop his Stamina...