Job Performance

I never found anything very difficult but achieving goal within a short period was a bit difficult is what I would say.

I would talk about my weakness in terms of technical skills. This question is a trap to see if the candidate fowl mouths their previous company, or managers or colleagues.

If you are a QA or a Dev tell the interviewer how difficult it was to test or code an application because it targeted an industry you have no idea of. For instance, if you were tasked with testing or coding say materials planning application that requires domain knowledge and you clearly lacked it. Tell the person whom to spoke with to to get that domain knowledge and how you gained that knowledge.
Your aim should be to tell them how you had encountered a problem and that you saw it as a opportunity to learn something new and ultimately ended up doing a good job if not great.