Tips for Early Game
If you’re having trouble, there’s no shame in grinding a few levels out on earlier maps.
As far as general tips, my main tip would be to keep your party somewhat smaller and try not to overlap weapon types. Too many characters will dilute your exp gains, and you’ll end up with either a team full of meh strength guys or a couple strongish guys and a bunch of weaklings.
The advantage of sticking to 1 character per weapon type like this is that it both keeps your exp concentrated, yet allows for variation in tactics.
Once you unlock reincarnation, a good tactic to snowball out of control is to reincarnate your story characters every few chapters and then catch them back up in levels. The mana cost to reincarnate doubles every time for generic units, but remains 100 forever for uniques.
In the end, stats are far more important than raw levels, so just a few extra points at lvl1 will snowball into a difference of several levels worth of stats later on.