now that I have 100% the game, I want to share a "passive" build I used for some of the more difficult bosses, hopefully this will help those who are stuck at these bosses!

Abilities:
1. Shackled Beast - Chain Whip
2. Lito - Rocket Fist (at least lvl 2)
3. Muninn - I recommend Stun, but Seeker or Bombard works too
4. Hugninn or Lorna, up to preference
you also want to equip a carapace - I recommend Impact or Pyroflective
for relics, I recommend damage reduction + cooldown reduction + carapace counter damage boost + auto damage boost, plus whatever else suits you

with this setup, the goal is to focus entirely on defense - dodging and countering - while all of your abilities combined deal a steady amount of damage over time, eventually whittling the boss down
Shackled Beast provides more counter frames, can be chained with carapace counter, and his attuner art counter deals *a lot* of damage, which is good for finishing bosses when they're at low health
Lito's Rocket Fist, at lvl 2 and 3, absolutely shreds at melee range when all 3/5 projectiles can hit and ricochet, it also has the lowest cooldown time; Lito can also be used in conjunction with other abilities, making him a very good at dealing damage without interrupting your action, thus putting you at risk
Muninn's Stun inflicts the Shock status effect, which reduces the damage dealt to you, giving you just a little bit more leeway for mistakes; if you are confident in yourself, Seeker or Bombard deal more damage
Hugninn is better if you focus more on dodging, all of his abilities are pretty much the same
Lorna, despite being an "auto" ability, actually requires a little bit of input to position her, so it's up to you if you prefer her or Hugninn for more passive damage output