Initially it happened to me too, but I saw that if you switch it at the right moment with another monster you can barely win without it dying. This happens very often, on calamity, for me. I put a "shield monster" to take most or all the dmg, I keep a strong dmg-dealer in the back and then, when the shield one is going to die, I switch them to avoid that. The poison is hella tricky because you have to switch them before, or divide the dmg.
I hate the little poison monsters. They are the worst XD
Very often, I find them more difficult than the "elite" ones.
Are there monsters with which you don't have this problem? Other bulky ones? Maybe Woad with a supporter that heals it? Healing is another strat I found that works well against them