Also you should probably end up using the story characters and choose a secondary class for them so say Ramza would be your knight with mettle as secondary then your some other main character has there special class and say time magic and then another character has his/her special class then black magic as secondary
My name is a secret because... It's just cooler that way
In order to find out good attackers, support, and range. Your going to have to experiment with different builds, and figure out what works for you. There are lots of different builds that are effective, but it comes down to what suits you best.
I would suggest at least 2 people with heal and revive as backup. Items, White Magic, and Martial Arts are all good choices. I would also suggest having some supports like Art of War, Speechcraft, and Steal, as they can be helpful as well. If you going for ranged characters, I would suggest Dragoons, Summoners, Black Mages, Mystics and Orators/Chemists with a Gun. The Monk and Samurai have some decent ranged abilities as well.
Once you start unlocking unique characters, they have powerful abilities. With your boosted movement, you don't really have to worry too much about closing the gap. The truth is most of these abilities break the game in how powerful they are, so in the end the enemies really don't stand a chance.
--- Life is hard, deal with it. Life was never meant to be easy.
Only random battles will be harder since they level with you. Story battles should stay the same since they have a set level. Correct me if Im wrong. Also this game isnt that hard. Im near the end of the game and I have fought 2 random battles at the beginning of the game cause I spent all my money on generic characters. Some of the enemies are double my level right now but Im still destroying them. Also like someone mentioned, keep a back up save especially when it asks you to save and youre in a castle.