Make sure to make use of the 'Power up' system in the Party menu back at the home base - stats are pretty low to start, so you need to boost them up with cash. Levels don't effect your stats directly, but you can get some freebie stat boosts (and skills) by using the 'Class Change' system to upgrade your classes at levels 5 and 15 each - and on top of that, money and xp are boosted when going solo in fights, so there's incentive to take advantage of that to mitigate the grind a bit.