how to prevent merc from getting one shot from her?
Im actually not sure. Because normally you would add PLR features, but since that poison spray your merc died vs is constantly reapplied poison damage as long as merc is in it, its not 100% as usefull, but should still help vs the dot damage after last damage was applied.
If you feed merc antidotes. 10 antidotes last for a long while, and it should maby bring res up to 95%.
95% is half the damage taken compared to 90%.
If you find some kind of 75plr it would help some too.
Gul rune is also a thing. (5% max pr)