A couple small suggestions to tidy up the main mission line bosses

Snarbolax
-Fix the bell not stunning the boss at a certain range
-Slightly increase the bell stun radius
-Give the boss more frequent opportunities to approach the player - less frequency on burying underground
The bell bug alone is very annoying, so that alone would do a lot.

Royal Jelly
-Make the small slimes be not immune to any multi-hit piercing attacks e.g Brandish/Magnus lines
-Make certain attacks more telegraphed (particularly one where you take damage randomly while at point blank range)
Doesn't really need any significant changes IMO, the non-piercing thing is just frustrating and unsatisfying (also leads to things like Drakon's Firestorm not working)

Roarmulus Twins
-Give them more of a "fixed" moving pattern e.g the vulnerable twin should always move toward or in the path of the row where the other one shot
Currently unless you do it optimally (sometimes even then), the fight is entirely RNG dependent on where they go next - it could take you either a minute or 5 minutes to do the whole thing and having to wait for the opportunity isn't very fun

Vanaduke
-Remove the multi-hit piercing immunity, likewise with small slimes in RJP
-Decrease the amount of fireballs during the 3rd and 5th phases and/or significantly increase/remove their respawn rate
-Increase the distance between both the fireballs and the boss itself (so at point blank range you wouldnt get hit by one)
-Alongside the changes above perhaps bring back the 3rd fireball row for last phase
The only viable weapon choice is the Blitz Needle.
Make room for more competitors - without the piercing immunity the Magnus line weapons like the Callahan/Iron Slug would be perfect viable choices, without the fireballs practically preventing anyone entering melee range not only it would make it more friendly for melee only builds but likewise we would have more varied weapon choices like Calibur lines (thats particularly important because the whole cobalt line is literally given for free), WRH, DA etc etc. Locking the fight off to a single weapon type is not good design and those changes alone would freshen this fight up significantly.