Sirens can be found on coasts of Skellige.
They are weak against Grapeshot, Hybrid Oil, Igni and Aard
On the ground, however, they are virtually defenseless, and so a wise tactic is to damage their fin-like wings to force them to land. The Igni Sign also proves effective when fighting against them. Threatened or injured sirens will let out a terrifying shriek, leaving their opponents stunned while they escape — and their sisters swoop down for an easy attack.
Much like harpies, sirens are easily bested once they're brought down to Earth. Some are more dangerous, though, in that their swooping attacks can poison you if they connect. It's also worth noting that if they're not killed quickly while they flounder around on the floor, sirens can let out a cry that stuns, giving them time to once again take to the air
Needless to say, your crossbow is a good companion to have whenever a siren is gliding towards you. Similarly, a well timed aard blast will also knock the monsters out of the sky, while igni burns the creatures rather well.