Proposal
Boss HP should be treated as its base HP for the purpose of applying status effects regardless of how high their HP gets from having multiple people fighting them. That’s all, it’s small but impactful.
Details/background on your proposal
If you didn’t know, a some status effects in the game require you to do a certain amount of damage to apply them. For example, Bleed requires you to deal 5% damage on most attacks that can apply it in order to actually apply the effect. With this suggestion, having bosses increase their HP based on how many people are fighting them wouldn’t affect the threshhold to apply bleed to them. Elius for example needs to take 60 damage from an attack to take bleed from it, but if you have 3 people fighting him he suddenly needs to take 180 damage from an attack to get bleed applied. Argos needs 150 damage taken in order to take bleed. Have two people fighting him and it takes 300 instead. With this suggestion, Elius would always require 60 damage and Argos would always require 150 damage taken to apply bleed.
Reason to add/change
I was fighting Argos with a friend earlier today, and they were a weapon build. They should have been applying bleed (which would have made my magic stronger) but since Argos had 6000 HP from there being two of us they weren’t applying bleed. It doesn’t seem like much on its own, but what about people with builds which rely on bleed to do more damage such as lightning or crystal conjurer? This change would make them more viable in group bossing encounters.