Is this “Siren” creature balanced?

I just finished playing a 6th level 4 players session and the GM used this creature in a fight with us:

Specifically it used the ability:

Sing (5/day). The siren releases a debilitatingly beautiful song. All who can hear it within 120 feet must succeed a Charisma saving throw of DC20 or be stunned. At the end of each of its turns, and each time it takes damage, the target can make another Charisma save. On a success, the condition ends and they become immune to the siren song for the next 24 hours.

We have two CHA based players in the party, so they managed to eventually snap out of it, but me and one other player couldn’t even make the saving throw because we have -1 to CHA.

The ability seems insanely overpowered to me, but at the same time without it the Siren would be useless. Is this creature badly designed? What would be the appropriate DC for this throw?