Poxrata is grouped with her friends Brudu, Grudu, and Krudu in the Tide Wall room ( -38, -10, -148 ) /waypoint -38, -10, -148.
Make sure you have a water breathing spell or charm before starting this fight. The Fishbone Earring from the Heritage Quest Hadden's Earring works perfectly, although some other water breathing spells such as Watery Respite may not.
You may need to focus on Brudu, Grudu, and Krudu, ignoring the red that Poxrata emits. Poxrata will be mostly dead by the time you have both companions taken out. Alternatively, if you have plenty of DPS, you can simply burn Poxrata, and her friends disappear when she dies.