The phrase "a difficult one which" is correct and usable in written English.
It can be used when referring to a challenging situation or task that is being described or elaborated upon.
Example: "This is a difficult one which requires careful consideration and planning to resolve."
Alternatives: "a challenging one that" or "a tough one which".
It's a focused challenge, but a difficult one, which helps rally governments, employees, parents, and customers alike.
It should be obvious that the notion of plausibility is a difficult one, which strongly invites further analysis but which is not easy to analyze in technical detail.
(Exactly how the constraints of the conversational setting, like the constraint to economize, is to justify governance by the DR is a difficult one, which cannot be undertaken here).
Differences in reporting, study design and the overall availability of data have made this task a difficult one, which is apparent from the results presented herein.
This choice was an extremely difficult one which was balanced between my love of service to our great nation and the need to concentrate on healing the wounds that I have caused to my wife and family.
The journey to diagnosis is a nebulous and difficult one, which reiterates itself in a lack of general awareness of the condition.
And, of course, there's that most difficult one, which is how does one be a good friend to one's self?" Why do we love "selfies"?
This was, he said, an "interesting and debatable" question, but by no means the most difficult one, which, in his opinion, was whether the fund should exist at all.
Difficult ones, which will need a lot of work.
The genre can be a difficult one in which to portray aging.
Still, the transition from exporters of primary goods to producers of manufactures was a difficult one in which the region participated unevenly.
