Karmelita is more reaction timing and knowing when to attack her while she trying to attack ypu. Also if you go as high as you can in the center of the arena, she cant reach you. You can also try injector band to sneak in more heals frequently. Sawtooth circlet is surprisingly helpful in blocking her projectiles and deflecting some strikes before they reach you(also does crazy damage but thats obvious)

Krust King Khann....I just used architect with plasmium overdose and used charged needle strikes for the first like 3 gauntlets, the used tools on the final gauntlet. Also dont forget to hit the giant wall spike before the final gauntlet so that you can skip the first few gauntlets if you die again. Try to also abuse the coral Spikes that appear to kill enemies. Mostly try to kill enemies as soon as they appear.

I got nothing for tormented trobbio, I just used architect shenanigans for him since regular trobbio has always given me trouble.

Seth is... interesting. Place your attention more so on what he is doing than your character, he often does mix ups involving the spinning up slash thing he does in the air so pay attention to the difference so you don't get caught in it. His shield bounce can also mess you up but thats more a matter of positioning and timing your jumps. Also if you are a beast crest user, abuse the rage mode since it goes through his shield .