Not sure if this will work for anybody else but I figured out a way to kill Pistrix even though he wasn’t swimming around. The skill challenge sign was above ground but he was still under the top surface but basically stuck in the rocks. clear out any mobs that might be near and equip a ranged weapon. (I used a rifle) Auto attack and it “should” auto target the mob but it will most likely say “obstructed”. Stand right on top and move around until you are able to shoot through the ground and start doing damage to him. Once he is dead it should give you the skill point! Hope this helps!