Hello
In my play time it’s been my experience that encounters with Beasts of Nurgle and Plague Ogryn bosses in particular can be pretty anti-climatic. For a BoN, If the encounter is on a part of a map that is open; then players that have aggro can easily stay far enough away while the others can easily get behind it and kill it. While with the PO, it’s attacks are relatively easy to avoid most of the time; so when there isn’t a ledge to toss players over to their death or additional enemies, a PO is not that dangerous.
Perhaps give the BoN a passive where damaging it imparts corruption damage, but the aura is only active conditionally or on an interval or something. So, a player can time it where they can hit the BoN without taking corruption. That way a BoN encounter would be more complicated than simply “get behind it and empty your weapon’s clip into the weak spot.” Since the BoN is a deamon, (a supernatural monster) I think a passive like that fits well. I also think it’s good because an aura that imparts corruption is something that is tweakable with difficulty level.
As for the PO, I don’t think a faster, harder to read, attack pattern is necessary (that is the Choas Spawn’s niche). Perhaps just having all of its attacks ignore blocking; i.e. players have to dodge all attacks to avoid damage. Or maybe have more attacks that send people flying in addition to the charge attack.
I would also like to suggest that ,if possible, the game tries to spawn specific bosses on parts of a run that gives that boss an advantage. For example, always spawning a PO with a horde ,or where there is a bridge; or a BoN in a tight space where it is harder to get behind it.
Thanks for reading