From my observations, the Harpy sisters are kind of like 2 "Plaguebringer"s, except for the fact, that each one puts only 1 damage over time on your pets, that increases with time, until they are defeated, or the other harpy sister puts her damage over time on your pets. Soo.... you have to beat them at the same time. Even though Freya has another ability that does some direct damage, and freezes the target of her attack for 1 turn, there is a counter to this. Nova usually casts "Heat ray" which adds a 3 turn, 25% damage resistance reducer/debuff called "Radiating heat". This debuf, even though it reduces damage resistance, makes it so when Freya attacks 2 turns later, with "Frost Nova" (freezes one sylesti), the freeze/stun does not take effect. It still does direct damage though. So when you see a little redish/pink person figure in your debuf slot, move that sylesti to the center, so no other sylesti get frozen.

Other than that tid-bit of knowledge... it's just spray & pray the damage. They aren't super hard, if you follow that procedure.

Thanks for all your posts! They were a big help in making it through the arena!