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.
Here is a video showing exactly where she is. It's easier to just watch the video to find malena but basically go away from the waterfall along the path leading back to the village, then make a left along a SHORT path, then make a left again on another path that heads south west sort of. You'll see a cut scene with some guy asking for help, then you fight some monsters, keep going up by the broken wall after you fight them, she is hiding behind the wall, watch out for the traps.
Best Answer
You can double-click on quests in your journal to switch them to the tracked quest. Sometimes (but not always) this will label your target objective on both the minimap, and the map page (default: m) itself.
The answer to your question, is Flotsam's Castellan (I think that's his title?) He's on the 2nd floor of the house directly to the left of the eastern Gate out of Flotsam (with Geralt facing towards Lobinden). He's also the one you talk to to complete the Nekker Contract, and he's involved with the first part of the Mystic River quest (finding a royal postbox) as well.