The two most important classes you'll be dealing with are Light Armors and Lancers (you'll get 3 of the former and 4 of the latter). Light Armors can raise how many squares you can move, so put them with any parties who lack good field attacks, such as Heavy Armors and Knights. That way they can get closer to the enemy faster. Spread them out among three groups. Lancers should also be spread out among another three groups (keep the fourth in reserve and swap him out when the others' levels are higher than his) because these groups will be slower and hence will have less chance to raise EXP. With Lancers, just use Javelin on the field for (almost) free EXP!

Your Summoners should always be together, and I recommend putting 3 Wizards together as well. You shouldn't need more than 2 Priests together, though. Instead of putting all your Heavy Armors together and the Knights together, stagger them, so you'll have both Defense and Critical in your melee-heavy party. Whenever you do use Mini-Devils, you should put all 4 of them together, too. When you get your Assassins, you might want to spread them out to allow more units to use them, or you might want to put them together to have greater effect for their spells; you decide. For rare battles where you'll want to be affecting terrain a lot (e.g. the one with the huge cannons) you may want to put a Wizard in as many parties as possible.

Always do swapping; you'll eventually have a bunch of characters in reserve, so once their levels end up being lower than the levels of your active parties, swap them. This way you'll keep everyone's levels satisfactory and you won't have anyone really overpowered. However, you should always have both Cross Knights, all three Light Armors, and both Summoners/Dragnars in every battle.

Dragon Association: Be smart. Put dragons with high Dexterity with Light Armors, dragons with high Thunder with Matelite, high Strength with Knights, high Vitality with Heavy Armors, and high Mind with Mini-Devils (if you use them). To put that another way, you'll want to feed your dragon Dexterity-raising items first if they're associated with a Light Armor, Strength-raising items if they're with Knights, etc.