The tough part about generating an argument against Existentialism is that any potential critique can be labeled as "bad faith," the Sartrean idea of wanting something to be true even though it is not. Existentialism is the only element that escapes bad faith because it concludes that there is nothing. With this in mind, I would suggest that one of the fundamental challenges in Existentialism is that it tends to "deify nothingness." If Existentialism is right in that human beings are alone, existence precedes essence, and that there is nothing that binds, then there can be no overarching totality. There can be nothing transcendent because there is nothing. The Existentialist might deify his own nothingness and thus view it as a form of totality. If the Existentialist constantly believes and thus preaches their own condition of isolation, this might move into a realm of making it transcendent, something that applies to all human beings, and thus rejected by the premise of existentialism in that nothing can apply to all human beings because there is no transcendence. I think that this might be an argument against Existentialism in that it puts the individual into a box, which by the philosophy's own definition, does not exist.
thank you very much now I understand the concept.