How would I ask someone "How difficult was that?"
¿Qué tan difícil era?
¿Cuán difícil era?
¿Cuánto dificultad tenía eso?
To ask about the dificulty you can use any of these forms:
As for the options you suggest:
This question is not gramatically correct to ask about the difficulty (it's more like an exclamation to express that the you don't think that it's as difficult as the other person said). You could say "¿Tan difícil era?" for something like "was it so difficult?", so it would be used when you already know that it was difficult.
It's correct, but "Cuán" is not very used nowadays, specially for questions.
Here you were close but "dificultad" is a femenine word so "Cuánto" has to agree with it in gender so it has to be "Cuánta".