[Rune Son of Bjarne]

What do you mean by "she got lucky with Ungoliant" ?

I doubt that Glaurung would be able to pass the gridle of Melian, in fact the only one I am sure could do so is Morgoth. Melian just seem like a very powerful Maia. She mannaged to lift the spell of Morgoth from Hurin and that does not seem like an easy task, I doubt that any other could have done it. (amongst the Noldor and Edain in Beleriand)

I only remember two ever passing the gridle and that was Beren which Melian predicted would do so and then Carcharoth, but I don't think that Carcharot would have been able to do so with out the Silmaril.

Maybe a host of Balrogs could have entered, but I doubt that Morgoth would risk an assault has he would have had very little knowledge about the power of Doriath.