Library levels sorta vary because some characters (byakuren) are a lot more expensive than others. But generally you should be sorta close to your level. Or well past it, on more important or cheaper leveling characters.
The first few bosses and the last few are the hardest, when it comes to the refights. The paralyzing wasp is also DTH'able, so you should probably just do that. After that most of them aren't a huge deal; the main issue is you have to get your buffs up IMMEDIATELY because it's the difference between handling attacks fine and your tanks getting one-shot (especially so on poison wasp). Even properly prepared, yes, it will do that; it's fine once you can get started without big losses. After that, the only thing I think you can DTH is the Glowing Azure Giant, and it isn't actually all that vulnerable; but if you just set up speed-build yuyuko in the front with Reisen and spam gameover-resets until it works, it won't take long.
Also, use Hina. Like, against every boss; all of them. For Biorhythm. You can make due with other debuffers instead if you've got several ones in your party, but Hina will make everything easier. Postgame can be very cruel without those debuffs and buffs up; even if you're surviving, people who don't do well at piercing def/mnd (whether through formula or sheer atk/mag) can struggle to hit decent numbers on a lot of fights, even good attackers like Kasen, and most bosses can have their offensive stats lowered after a few casting attempts which will make a world of difference.
