I'm hoping to update this and eventually turn it into a FAQ here on Giant Bomb, so if anyone has any tips or advice, please feel free to chime in.

General Combat Tips

Concentrate on weapon specialties at first

If you go into your status screen and press the right analog stick to the right, you'll get all sorts of useful and useless information about each character. One thing you'll want to pay attention to is weapon specialties. At least at first, it's a good idea to equip that character with his specialized weapons, as he or she will be able to gain skills for those weapons through the Evilty dealer. Which leads me to my next point...

Keep updated on your skills and/or evilties

Each character has different skills and Evilties you can obtain depending on your class or if the character is specialized (storyline-wise). I like to get a couple of basic skills, upgrade them once or twice, and then focus on obtaining the unique Evilties. Some of them are ridiculously awesome, like the ability to absorb enemy stats after defeating them (which is a temporary boost, but still useful).

For mage characters, I tend to avoid boosting their Evilties until I've got a good amount of range and power on their primary spells. It also doesn't hurt to reincarnate these characters several times when you've got the extra SP and have a powerful leveling area. Reincarnating a character allows them to keep their prior skills and learn new ones from a different class. All mages should pick up the Healer's skills at some point (though not a necessity, it really helps) as well as at least one or two other mage abilities like Fire or Ice. Please note that reincarnation is really not necessary if you just want to play through the main storyline first - this is all just hypothetically assuming you're going to get into the deeper endgame stuff.

Learn the basics of power leveling

This is an essential for making grinding far less aggravating. Basically what you're looking for is a level with tightly grouped enemies and a potentially quick win. Level 5-2 is my favorite so far, as it has groups of enemies packed together, sitting on one color grid with two +50% EXP boxes just off to the side waiting to be tossed down. You have several ways to power level here, but the easiest is to stack your characters to a height of 10 with your most powerful human character at the bottom. Then proceed to whip some monsters' asses! You can lift and throw enemies into each other to create more powerful enemies, which can be tricky if you're underpowered or don't have a lot of HP, but if you do, it'll level your weaker characters up fast, because they'll gain all the EXP from the kill without being in harm's way, as your bottom character will take all the damage for you.

If this isn't providing you with fast leveling for your powerful characters, you can go to the Senate and pass a "More powerful enemies" bill, which will make them stronger but also give you more experience for killing 'em. Just remember to continually save your progress if you go this route, as you can create some awfully damn powerful enemies pretty fast.

GB Member Drac96 Chimes In On Leveling in 5-2

"Also this is just from researching on forums and such. 5-2 is one of the best places to level. Apparently the exp is its best when you pass 8 stronger enemy bills. This makes them level 99 and creates the optimal experience. In previous games it was believed that this was a glitch that caused them to give the exp of level 300 enemies. I don't know for sure, but I don't think this is the case now. It has something to do with the experience leveling off from 99-299, so you'd only get more exp when the enemies are 301. The method I use on the level is have a runner like my thief go to the right and throw the block near my base. Then I have another character break it. This makes basically the whole level 150% exp through a geochain."

Thanks Drac96!

General Gameplay Tips

Item management is crucial in the Item World

Always, always, ALWAYS keep an Mr. Gency Exit in your prime inventory if you're going spelunking in the Item World. You can use them at any time, so when you're facing some tough enemies, always keep one character in reserve to bail you out in a jam. All he needs to do is pop up, use the item, and blammo, you're home safe.

Now, other item management tips. I really recommend passing as many "More Expensive Item" bills as you can afford in order to keep a better stock of higher end healing items on hand. You'll still usually get the basic healing items in the store as well and you can always pass cheaper item bills later if need be. Keep your primary inventory stocked full of healing and SP recovery items that will heal your most powerful characters and forego keeping anything less powerful. Unless you've taught a few characters Espoir (the game's status-curing skill), you'll want to keep a couple of Fairy Dusts as well as a couple of high-end thieving items (if you have a thief with a reasonable level). By keeping your primary inventory full of useful items you can actually use, your loot will go straight to your secondary inventory, which can carry far more but is only accessible in peaceful areas (i.e., your main base, Mystery Rooms, and Item Town). Always remember to switch out junk you can't use with stuff you can in those peaceful areas so that you're never caught with your pants down.