@boomy
Okay I'm an idiot, nevermind. I was missing the SRPG_BuffStatesCore.js this entire time. It works...except the damage popup is delayed for some reason? The enemy moves to me and then suffers the damage over time from poison. Also if the poison kills him, he kind of just vanishes without showing the damage popup at all.
Instead I would use <Custom SRPG Phase End Effect> which runs when the battler's phase is over.
As for pop up, does using a.startDamagePopup(); after a.gainHp(-10); help?
As for poison killing the enemy, that's technically not a plugin error but just how states work
You could do a check:
if a.isDead() { a.performCollapse(); }