You asked the right question of the right person

To start with, you MUST get a clear picture in your mind of how you want to level up your characters and what you want them to be like in battle. If you don't they'll never be any good at anything.

Martyr covered a lot of good things that I won't re-hash, but I can't stress battle tactics enough. Remember that the more things you try to make someone do, the less good they'll wind up being at it. Making Frioniel a Paladin isn't a horrible idea, but you'll have to effectively double the training you do.

Another bit of advice I can give, and this isn't something the game wants you to know about, but magic and equipment HATE each other. Armor with higher defense cripples your evastion and magic abilities by a LOT. Weapons that do a lot of damage are just as bad. If you make someone primarily a magic user, give them weak armor if you want them to be any good at magic. Trust me, this WILL make a difference!

I could basically turn this into a VERY long essay of a post, but I'll spare everyone else that. If you want some more advanced strategies, PM me.