The normal harpies you can toss the fire bomb item at them and that usually one shots them, or you can toss a flashbang to avoid their fight. They're weak to fire so you can two shot them with magic that way. The boss harpy is another story and pretty annoying/RNG based. My strategy was to survive all of her attacks by defending and spamming the wind/AGI buff, then when she tries to eat you with her tentacles you can attack her for extra damage, only you have to pray her attack misses twice thanks to your evasion. Being hit is pretty much game over. There's probably easier methods I'm sure. She has a lot of HP so it's a battle of attrition this way.