Help with Mamba DMG/Poison

https://pobb.in/nOQhq9W93PRH

I don't know why, but i barely deal dmg to wave 11/12 bosses in simu.|
Seems like i don't cast poison on them.

https://preview.redd.it/z6snpvxdhaaf1.png?width=582&format=png&auto=webp&s=70ef9d8d6f2de858a9dae9b1b537583ec6f0607c

Here's my merc. Even with her or without her, i seem to be stuck on those waves.
Rares/Bosses also take quite some time to deal with.
What can i do to... change this build to more...reliable one?