Poison used to be a good cheese vs immunity shields. Easiest way to kill quov tsin (when he was the hardest boss in Median) was to roll a hpally with plague, get killed and get lucky applying psn damage to it during getting killed. There were some loopholes in the engine that allowed it even though the boss had 1000% poison resistance - total immune, impossible to break this immunity in any normal way.
And plague back then used to deal over 1 million damage. It had a huge duration so DPS was still rather low, but it worked with this way of killing bosses. You just waited dead while boss health dropped, eventually reaching 0. Release, pick up loot.