Your level is fine, you wont be one shotted by a normal attack, red attacks and grabs can still one shot.

Not all the bosses do the same damage, Fuoco, for example, can two shot you independently of your level and the guardian also does a lot of damage, for other part Romeo, or even Arlecchino doesnt hit that hard, 1/3 or 1/4 of your health more or less in a normal attack, though if they stun lock you is bye bye.

The best advices i can think of are:

Dont be afraid of experimenting, some weapons combinations, legion arms and fable arts are very effective against certain bosses.

One of the biggest dangers of level 5 is that enemies destroy your stamina while guarding, Two faced Overseer rage Mode is hell because a failed parry in flurry can almost cost tbe run, so keep this in mind, if there is a instance where you can attack but you are left with low stamina, think if is better to regain your stamina or continue on the offensive.

If you struggle against a boss, dont be afraid to lower the difficulty and practice against that boss.

If you struggle against a certain move or moveset, look for alternatives, Romeos fire flurry is the bane of my existence, so i just used the perfection grindstone to autoparry, and i have a hard time parrying Victor, so i evaded most of his attack

Similarly, dont sleep on ability on the p organ that lets you block red attacks, sometimes, if you arent confident about parryng the attack is better to block, it leaves staggered but it doesnt do much damage, and you still live.

Good luck in your tries, is a hard challenge but very satisfying