The phrase "and infinitely more difficult" is correct and usable in written English.
It can be used to emphasize that something is not only difficult but to an extreme degree, often in a comparative context.
Example: "Learning to play the violin is challenging, and infinitely more difficult than mastering the piano."
Alternatives: "and much more challenging" or "and far more complex".
Behind the debate on women bishops, there are unresolved questions about the ministry of gay priests and the infinitely more difficult issue of same-sex marriage.
The trouble with much of the advice business is getting today about the need to be more vigorously creative is, essentially, that its advocates have generally failed to distinguish between the relatively easy process of being creative in the abstract and the infinitely more difficult process of being innovationist in the concrete.
"Making it go further and become more practical is infinitely more difficult, and that's down to the basic physics of how batteries store energy and release it".
Writing a memoir is ostensibly easier (you are automatically the leading authority in the field!) and, for this reason, infinitely more difficult.
The mile and 10,000-meter double was infinitely more difficult, he said, which would explain his reaction after he dragged himself out of the water.
Finally, embittered with the knowledge that dealing with party members, each with his own constituency to answer to, was infinitely more difficult and distasteful than working with a handful of genro, all of the same background and inspiration, he resigned as president of the Rikken Seiyūkai in 1903.
Getting started as a new tech company is hard enough in this highly competitive landscape, but actually staying in business and achieving sustained growth is infinitely more difficult.
Rosa had the infinitely more difficult challenges of racism and segregation to confront both growing up and throughout her life.
UN System fellows, on the other hand, return to different organisations and the impact of their learning on their home institutions is infinitely more difficult to assess due to limited control over their deployment and support once the fellowship is completed.
