The thralls appear to be linked with her, when I tried to pull her single and feign the others off they all came running back instantly every time. Likewise, when I tried killing the outlying thralls singly, they all seemed to aggro instantly. I found the best solution (after a few deaths of course) was to do exactly what the poster below suggested and pull her to land. I aggroed them all, pulled them to land, and was finally able to separate Mirlil on dry land. Her effect is annoying, but much less so when your out of the water.
She can be pulled to land. otherwise she will fling your group in and out of water and you will get adds unless you clear all around her first. We found it easiest to just pull her to land and get her solo.