How do I avoid the Queen Endrega's poison and kill it?

I just killed my first Endrega Queen (I find out we have to kill two of the things. TWO!), and if you're getting hit by the poison, you're probably fighting much too close to the queen.

Hit and run tactics are your best friend here -- the greater distance you are to begin with, the easier it is to dodge the poison stream, the charge attack, everything.

The other useful thing was the Aard sign. The damage / daze effect it creates can stop most of the Queen's attacks mid-animation, giving you time to get in a swing or two. Dart in, blast her with the sign, strike a few times with your sword, dodge away, wait for vigor to recharge, and repeat.

I found that when fighting the queen, the quick attack style oftentimes did "minimal damage", so stick to heavy attacks when you can.

The last helpful advice I can give is that the Queen shakes her head just before spraying the poison, and rears back before her charge attack.

I found that the most effective tactic by far was to always stay behind the Queen Endrega. It takes far, far more damage that way as most attacks from the front are blocked. The difference is really big, attacking from the front I only made a very small dent into its health bar, attacking from the rear I halved it in a few swings.

I was able to get behind it by rolling around (I had the first upgrade that increases the distance you roll).

There are also potions that increase your poison resistance you could take before the fight. Golden Oriole gives you +30% resistance against poison, incineration and bleeding.

If you've been collecting the snares from the forest around Flotsam, they're the easiest way to kill the Endrega queens. You can stack the snares on top of each other.

Destroy 2 of the three cocoons. Then lay 20 snares on top of each other. Then destroy the last cocoon to trigger the queen. Lure her onto the snare stack and she'll die instantly.

It's anti-climactic but VERY effective. Just make sure you don't walk onto the stack or it will instantly kill you too.