FFT storyline was like a damn Shakespeare play. The best thing to do is go to Amazon and get the Prima strategy guide. Whatever levels you told to be at, be at least 10 higher.
What I do at the beginning of every game, I train Ramza, etc., to be at least 10 levels higher than what the strategy guide says. I be at level 15 before reach Dorter Trade city the first time. Go to random battles in Mandalia plains and learn the basic job classes like chemist, knight, thief, and accumulate JP and gil to buy more phoenix downs and potions. Then the Story battles be easy as fukk when you 10 levels above your enemies.
For chapter 1, Let Ramza be a Knight and go to random battles with him and his core team. Build him up and make sure he learns accumulate strength +1, Gained JP Up, and Move + 1 from Squire job class before you become the knight. Then in every turn, he can use the accumulate job ability and build his experience points along with jp. Make sure his team builds up with him too, like the Chemists and their abilities to learn Phoenix down, the warriors learn that too, and so on. You should have 2 Chemists in the building up session and make Ramza have "Item" as his secondary ability. One of your other offense/ warriors should be an archer to build up jp points to have access to thief class. Once you learn several abilities from the Knight class, temporarily make Ramza into a thief and make him learn Move +2, steal gil, and steal weapon, steal armor, and so forth. Make his Guts ability into the secondary ability temporatly and use accumulate or steal gil constantly to build his jp points and build his jp to purchase those abilities. Once you real level 5 thief for Ramza you should have 1200 jp to buy Move + 2 and to learn steal weapon and steal armor. Once you do, turn Ramza back to Knight and build him up, with the rest of your team, to have numerous abilities in the Thief Class. Have you other warrior into a thief and let him stay there for a while so everyone benefits getting jp for the theif class.
May be overwhelming, but at least you have mad gil, jp, higher levels, and so on before you reach Dorter Trade City in chapter 1. Make sure before you end chapter 1 with being over level 30 for all your team members
By Chapter 2, automatically switch Ramza back to a Squire, cause by then, they can equip heavy armor and shields like a knight. On top of having Move + 2 from the thief class in his move ability, and the gained JP up, On top of his Cuts ability. You be moving all up and down the battle field with no problem (Move 6 on the battle field is

) and you be super strong.
I got more strategies for FFT for anyone that wants them.
I appreciate the strategy, but you working too hard levelling up my dude.
Dorter Trade City is the first difficult fight of the game. However, all you need at that point is a black mage with a level 2 spell, a white mage, Ramza as knight (my Ramza build is very close to yours actually) and an archer (who will later be your lancer...an underrated class imo).
And you don't need to be at a ridiculous high level either. I routinely beat Riovanes entering that story sequence at level 22 with Ramza.
My basic strategy is to first of all reset my first save until I get males with high brave and females with high faith. Then I turn everyone into squires and learn gained JP up (and move +1 for the males/fighter classes), then turn the females into chemists, learn potion and phoenix down, then turn one female into a wizard and the other into a priest.
The wizard will eventually become my calculator and my white mage will eventually become a pure magic caster, and switch from priest into wizard after learning cure 2, esuna, and life. Then as wizard learn magic until having a level two spell, then back to priest to learn cure 3 and holy, then back to wizard to learn a level three spell. As you're doing this, you should be collecting extra spells/abilities from crystals in story battles with other mages. By Riovanes, you should have a calculator with all of the multipliers, haste/slow/stop, black magic up until level three, white magic until level three, and Holy. Also, take time to give your calculator auto potion to help keep her alive. Once you learn all the skills you need from calculator, then switch back to mage classes and learn all of the spells.
Your white/black mage should only stay in wizard class to learn spells, but spend most time in priest class since priests have higher speed than all wizard classes and most fighter classes. Basically, you want your priest to be able to cast protect/shell and any curative spells before your enemy has a chance to kill you. Oh, also don't forget to get your priest short-charge--fast cures and Holy makes you so much more effective. During any fight where you bring your wizard, your priest should be in the same fight so that they can leech JP off each other. As a third mage, you can have a strict summoner/time mage--she doesn't even entertain other classes, just summoning with short charge and time magic. When not learning summon spells, stay in time mage class since summoner has the second lowest speed of any class.
With your two fighters, one will be a thief/ninja, and the other a knight/lancer. The thief should be in ninja class (after learning how to steal everything) because ninjas are really really fast and have incredible movement--they make effective thieves. Your knight/lancer should learn up to charge 5 in archer class since it does a lot of damage without charging for too long. Lancers also have great shield block stats, and can leave the battlefield by jumping. You can win a lot of tough fights with a lancer who can jump anywhere on the battlefield or use charge 5 when you don't have to.
With Ramza, raise him to be a Samurai, learn all of the draw out skills and blade grasp, learn magic attack up (draw out damage is calculated using the magic stat), put him back to squire, get move +2 (or move +3 if bard is available to you), and scream, and 97 brave, and Ramza is a dude that can catch bullets, and do 300+ damage with draw out while moving all over the battlefield with high speed. He pretty much can go where he wants untouched. =D
Then take Agrias, get her lightning stab, then get her teleport, then switch her back to her native class and master it.
In using teleport, you will basically have a character with move of 6 and jump of 4 at an 85% rate.
You can achieve most of this before getting to Riovanes without level grinding, if you have a plan.
With this build, the only moment during the game you will need to JP grind is to get the calculator skills before Riovanes (and teleport for Agrias before Riovanes if you really want it)--otherwise, just complete all the jobs at the bars and fight random battles in between. You'll be ready for every story fight in the game, and you will win, easily.