I've noticed that many artifact effects that were likely deemed too effective against single targets were set up in a way that they did nothing against bosses.  This is bad for balance, as it allows artifacts that are designed in a way that is not nullified to become extremely effective against bosses, while nullified artifacts become entirely useless.  Here I'll come up with ways to balance these artifacts against bosses.

On one hand, one might argue that changing the effect against bosses without saying so in the ability description is a problem, but on the other hand, it currently removes the effect against bosses entirely without saying so in the description; so I think this solution is still better.

Defense Reduction Effects: I suspect that effects which reduce defense are disabled against bosses.  I think that these effects should be re-enabled, and just scaled down to 1/5th of their normal extent.  For example, something that would have reduced the boss's defense by 20% will instead reduce it by 4%,which is still helping quite a bit if stacked.  This should apply to anything that reduces defense that was currently disabled against bosses, such as Suppression and Preemptive tactics, or Aylin's artifacts.

Stun Effects: Stun effects that are currently disabled against bosses should still apply 20% of their stun duration to bosses.  For example, a 1-second stun would stun the boss for 0.2 seconds, and a 6-second stun would stun the boss for 1.2 seconds.  That should be enough of a reduction that actively timed abilities cannot permastun the boss, and randomly calculated unit stuns won't time perfectly to permanently stun either; thus stuns will still be helpful, but people won't be able to abuse stuns by overusing it to permanently stun bosses.

Other debuffs, such as attack power and attack speed reduction, should also apply 20% of their effect to bosses.

Now, for some artifact effects:

All of Serath's Artifacts: Serath's artifacts cause effects for every 20% of health the target loses while the effect is active.  Against bosses, this will clearly not trigger at all, making the artifacts have no effect whatsoever.  I suggest changing these artifacts to simply add the mentioned effect once upon the use of the ability when fighting bosses.