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.
Vathsis spawns in the Sentinel's Lair ( 68, -8, -261 ) /waypoint 68, -8, -261. This is the room to the North of the center.
During the fight, Vathsis activates the four statues around the center of the room one at a time as an infused effigy as his health decreases. He starts with the one to the southwest of the center and sequences through them clockwise.
The infused effigies drain your power. They can be killed one at a time with slow DPS on Vathsis, or if you have a lot of DPS you can ignore them and just burn Vathsis, the adds disappear when he dies.