I hate to say it, but I don't believe there's any way for a plugin to reliably know which debuffs are on a mob, because of the flakiness and bugginess of the player's target's effects list. (Off the top of my head, I recall that the EffectAdded and EffectRemoved events are all wrong, and even ignoring the events and polling the list itself doesn't give you accurate information. I'm referring to the target's effect list; the player's effect list works fine.)
I submitted a bug report years ago, but it seems like one of those things that will never be fixed. (Plugins were never "officially supported", and the current skeleton dev crew doesn't have any time to work on Lua bugs.) Good luck. I'd be very interested if you are able to find a workaround.