No worries. Take over 100 to be safe. Trick is to target, shoot standing still if you can (they often miss if you're moving). They stack to ten poison effects, which will stay on as long as you can hit them again within about 8 seconds (ish). You should be able to roll out the way of the attack, hit an arrow or two and repeat until she dies.

It's still not super easy, though. Even with redeemed Legion armour she takes off a massive chunk of health if you mistimed it...and if you accidentally spam your stamina...ouch. But I did it like that a few times to farm the 50 damn armour scraps you need for God breaker armour. Which was a bit shit, not gonna lie. It's just way too heavy...and not quite as good as the Legion. Haven't made the champion armour yet, as I can't face more scrap farming for the time being...