Uhh I forgot, I think her water elementals aren't required for defeat? You just need to defeat the harpy herself. I think she's also resistant to frost damage but performs a lot of frost damage herself. Get a tank for her to aggro on and a kunai or other bleed effecting item to reduce her healing and nuke her. If you're a tank yourself, equip frost resistant gear. Have an AOE stun power if the water elementals are too obnoxious
This is just guesswork from my head, I didn't test and don't feel like looking at the game right now.