Helldritch
Hero
My players hate flameskulls and beholder. In fact, anything that flies seems to be their weakness.
I used a Banshee once on a party of 4. Three PCs dropped to 0 and the barbarian couldn't handle her alone and save the other PCs. He did get lucky with his rolls and drive her off, but not in time. One of the other PCs failed 3 death saves while he was fighting.Banshees are lethal. That wail sounds off and half the party drops to 0hp or flees in terror. Everyone else has to scramble to protect/rescue their fallen comrades from an incorporeal undead monster and her bodyguards. Brutal.
The blue slaad? A CR 7 creature, so mostly encountered between level 6-10 I'm guessing.