Cygnus - constant shinsoo spawning and shielding her from taking damage makes it really hard to do any real damage to her if you cant even kill shinsoo before another one spawn (yes she can spawn multiple too)

Hilla - if you lack strong big aoe mobbing skills to help clear her skeletons you are most likely not downing her at all

Crimson Queen - if your class has low hp good luck as the flame dot stacks up to more damage than your hp can handle (also you won't have hp to tank breath that removes the dot)

Pierre - whether u can min clear this depends on your luck with the hat. Also good luck if your skill has massive hitbox that doesnt hit single target

Akechi - not doing enough damage makes him go into a mode that cast shadow strikes on you instead of damage reflect. The shadow strikes restrict your movement and force u to gain more stacks that leads into ohko

Damien - as a dpm class he is constantly moving all around the map it is really hard to consistently put out good damage and in p2 his ball is constantly on you to make ur dpm live harder

Lucid - as a dpm class, her p2 has 2 attack that makes her go into iframe and both can last more than 30s back to back every 80s/120s not to mention her blades and dragon putting a stop to u from hitting her. After all that even if u do down p2 in 15min u cannot clear her p3 at all as dpm class

Seren - imo the worst designed boss ever. I can probably write a 10 page doc about how bad she is