The fundamental issue you're running into is inflexibility. You've created certain rules for yourself that you've decided you must follow at all times, even when it's to your detriment.

The people offering you advice aren't speculating on what *might* work. They're people who have killed the boss with no trouble.

Lots of Sages kill this boss every day. They do it by not running through poison. I would personally start by trying a strategy of not running through the poison. Even if you try it and don't kill the boss right away, I promise this first step will be necessary for you to get this kill.