Early in the game, you should train Ramza and four generic soldiers, who will do the bulk of your fighting in chapter 1.

Once you get Agrias and Mustadio, you'll only need to be training two of the generics. Eventually, you'll get enough unique characters that you'll never need to fight with generics again. But don't dismiss all of them if you want to do propositions!

I find that most unique characters work best in their default occupation with Math Skills as a secondary ability.

Also save some room in your roster for monsters. You can get some really nifty items by poaching them plus some make really neat allies.

Most bosses have fized levels and stats so you can always overlevel and overpower them. In random battles the enemies are generally the same level as your party.

My advice is to fight and train a lot in random battles between boss fights. Once the best weapons and armor you have is not working against random encounters, advance the plot. Generally after advancing the plot, shops will sell better weapons and armor.