I know there are mods that simply tone them down to manageble amounts, but there needs to be a method to fix this internally without reducing the challenge factor for clans and such. As it has been done in games before, they could introduce mechanics meant to be observed and countered which do bonus (or open a window to doing bonus) damage to them or a (assumebly harder) scale factor.
If its a burst of damage scale it based on the enemy encounter, something like a timed CC effect or luring/applying them into/with an effect. Skilled and creative gameplay benefits from this the most However in some cases it can be easy to exploit.
If its a window scale it should based on a percentage before closing the window such as a window of cc or a “stance” that inflicts more damage to them while in it. The reason for a cap is so larger clans cant over burst and melt the boss compared to smaller groups. The damage bonus would end up being the same just larger clans would have an easier time getting to that cap per interval.
The other option is scale damage/health/defence on number of players nearby however this comes with many issues. Firstly not everyone is built equally and secondly this often scales very harshly for larger groups.
Ive seen games try this and it barely works, so there would need to be some variation of the three to keep bosses challenging and balanced for various server/clan sizes. Ive seen this as an issue from the start and it still seems like an issue. I hope they address it promtly because difficult enemies is a big attraction for me in games that demand skilled play.