First of all, when people say you should “level up,” don’t take this to mean that you need to raise your levels. Levels are rather meaningless in this game for the most part. What really matters is your skills and classes. It’s easy to get to chapter 4 with your characters at an average level of 25 if you have the right skills and classes.
Also, if there’s anything I’ve learned it’s that there is no such thing as a class that “sucks hard.” Mimes are useful for raising your magical attack power. See, the magic growth rate for all classes is the exact same, except for the mime which raises the character’s magical attack power like mad. This is great for characters who end up using Draw Out as a secondary skill, since all the draw out skills are magic based. Archer is about the closest to the “worst” class but even they have the nice “Concentrate” ability which can make your physical fighters much more effective in the early going.
My favorite normal character class is probably the female Geomancer because of her good HP, her ability to wear equipment that raises physical and magical attack power, and the fact that she’s got good scores in both to begin with. Give her “Draw Out” as her secondary skill, and jack up her Physical and Magical powers to as high as they can go (and keep the scores as close together as possible) and she will tear people up.
I’m also really fond of the oracle and the time mage. They both need really high Faith, and really high magical attack power to be truly effective which is why I tend to have only females take on the role. However, the last time I played, I had my Ramza be an oracle with 97 Faith (because he will never leave the party due to his high faith, and his magic power is as high as a woman’s) and found him to kick righteous ass at all times. Oracles not only have nifty magic, but their sticks have a range of two panels and hurt like you wouldn’t believe. Draw Out was his secondary skill once I got him high enough, but until that point I kept him with his “Guts” skill because of “Yell” and “Cheer Up.” To make good use of Time Mages, you should pay attention to your own characters Zodiac signs. If they’re really compatable, you’ll have no trouble keeping your allies hasted (this is a very very good thing 
) and can potentially slow or stop your enemies cold in case you need extra time to steal from them, or need to keep them out of the way while you steal from their friends… or even just to keep them from killing a weak party member before you can get someone over to heal them or get them out of the way.
And I second the reccomendation of checking www.gamefaqs.com. Some of the FAQs have bogus information, but the “Battle Mechanics Guide” is your bestest friend in the whole wide world for when you have the time to truly experiment with all the options.