The phrase "a difficult one that" is not correct in standard written English.
It seems to be an incomplete thought and lacks clarity without additional context.
Example: "This is a difficult one that requires careful consideration."
Alternatives: "a challenging one that" or "a tough one that".
This role is a difficult one that comes with the demands of constant changes and very difficult decision-making.
As a sailor, the road to gold is a difficult one that starts with the dreaded Flight Selection Series.
Although he trails in nearly all national surveys and polls of most battleground states, Donald Trump still has a potential route to victory, albeit a difficult one that would require him to coax many people who sat out the last election to vote this time around, the USC Dornsife/Los Angeles Times Daybreak tracking poll finds.
He said he understood that the conflict was a difficult one that would not be solved by music alone.
The problem of identifying and annotating the functional cis-regulatory polymorphisms is a difficult one that will require various experimental as well as computational approaches to address.
It's a difficult one that we've never really solved.
The prevailing narrative about abortion tells us that a women's decision to terminate a pregnant is always a painful, difficult one that will leave her devastated.
The decision to address this issue publicly was a "hard, difficult one" that took a number of years to gestate, he tells me.
IME: Difficult one, that.
The task, an emotionally difficult one that draws little sympathy -- these are, after all, the survivors -- is immensely complicated.
And he is candid enough to admit that he should have realized that the war would be a long and difficult one, that democracy in Iraq would be unlikely and that the American administrator in Iraq, Paul Bremer, had led the country into chaos by recklessly and incompetently disbanding the Iraqi police and military, and the Baath Party government.
