What do I do about PCs using Con damage to “nuke” bosses?

So recently my players realized that they can spam poison-based Con damage on my bosses to effectively nuke them down in a couple rounds. This is basically ruining the challenge of my fights, but they seem to love it. I’m not sure how to handle this without basically saying “No, you can’t do that.” Or making the bosses suddenly immune for some reason. Their entire tactics rely on lowering the enemies saves, then spamming Con Damage to lower it more, which allows easier Con Damage spam, until it’s dead by round three.

I don’t know how to handle this. I don’t want to be an ass and just start doing it back in a “Well fine, if you do ima do it too” fashion.

This sucks…

Looking for an rpg game that is open world with bosses and loot tables [closed]

Recently I have really gotten into the rpg genre, but none of them are really what I’m looking for. I’m looking for a game that is open world, has good combat (not just spamming without worry of dying until everything is dead), possibly a skill tree, classes, an mmo so you can just find other people roaming the world and help each other and stuff, and a good loot system like uncommons and rares with possibly modifiers too(sharp, weak, legendary, etc.). I know it’s a lot and kinda picky but I just can’t seem to find something like that.