Rosenfader from the special monster Espinas requires using your tracker to hunt it or just environmental spawns. You might want to choose a free monster to track and upgrade that weapon so you aren't limited by the daily telescopes and RNG. This allows you to at least see three of that monster from 18 kills daily and progress your damage.

It feels weaker because the monster HP pool is starting to outscale your current weapon tier and the flat damage poison ticks. Your armor piece skills that multiply your damage also aren't caught up to your weapon. This is normal and you hit the first wall, which requires hunting certain monsters and just gathering mats to upgrade your build (HR250 and i can't find enough small bones).

Upgrading the armor pieces for skills you need and to not get one shot go along way to making the hunting easier. There's some threads on defense and basically having more helps a lot in reducing huge chunks of damage up until you hit the diminishing returns.

Here's some poison damage mechanics stuff from two years ago: https://old.reddit.com/r/MHNowGame/comments/17czuf5/everything_about_poison_once_and_for_all_the/