OK so its been said that Gamzee and Feferi are durable due to their high blood caste and even Terezi was shown taking multiple wounds before dying. So why was Bec Noir able to kill their dreamselves with just a sword stabs?

Because after that he attacked the moons with a First Guardian attack and incinerated all of them in green fire. Also I don’t think even a highblood like Feferi could survive being bisected.