Quoted from FatPanda:
The first boss fight that I'm encountering after collecting all of my members is *killing* me. It's Omar on Tressa's first main quest. I went at it for a good hour before he buffed and wiped out my team in consecutive 2 attacks! Even after grinding up to lvl 25 or so (recommended lvl 18 or something, right....), he's still giving me a hard time. I'm going to have to try Cyrus's quest again in the same town...I've never had an RPG kick my ass this bad since FFVII Ruby/Emerald Weapon fights!
I started spending all my money on weapons, then realized direct attacks (as in weapon with attack commands) are not what takes down bosses, Cyrus does heavy damages with magic once boosted at Max. So I end up spending most of my money on defense gear and use base weapons to break ennemies then all out on anyone with magic.
Also make sure you upgrade your skills, it brings powerful magic and strong attacks, I totally forgot about doing that while gathering the 8 characters, so I got thousands of points to use at once which make combats way easier.
I haven't been stuck on any boss so far, and come with at least one character at the minimum level shown but when I feel combat is too unbalanced I stop the quest and come back with the proper people or go on another quest to do some grinding.
My base team is Olberic, Cyrus and Ha'anit, the 4th spot goes to whoever quest I take. Otherwise I keep Ophilia to heal, although healing items are not too expensive to buy. Another thing is to take the time to complete side quests they reward you with some great gear and use Therion's steal to get some good healing items.
Boss fights are epic, but there's a clear shift in difficuluty between the first part of gathering the team and what comes next. I just love this game, and definitely aim at a 100% completion