"essential difficulty" is a perfectly correct and usable phrase in written English.
You can use it when you are describing a problem or challenge that is necessary or integral to a task. For example, "The essential difficulty of the project was that the deadline was too tight for the resources available."
There does seem to be the essential difficulty of grappling with any understanding of Scientology.
After the researchers plotted the data, they confronted the essential difficulty of all attempts to say just how frightful the weather feels to the average person.
The essential difficulty of monetary policy is that interest rates need to be changed many months before they have any effect on inflation.
4.09pm GMT Verdict Today's debate is complicated by the essential difficulty of defining exactly who is meant by "the workers": it reminds me of the possibly mythical tale of a poll in which 98% of respondents considered themselves middle class.
By Jia Tolentino June 8, 2017 Andrea Constand, who has accused Bill Cosby of sexual assault, has been a remarkable example of what it takes to stand up to the essential difficulty of the dynamics of a rape trial.
Though her cross-examination was scattered and slow, and, all things considered, probably easier than it might have been, Constand has nonetheless been a remarkable example of what it takes to stand up to the essential difficulty of this dynamic, in which an alleged victim, after being forced to participate in a sexual encounter, is implicated by the fact that the sexual encounter occurred.
The proof in this article follows essentially the ideas in [16], with some essential difficulties because we consider the new dichotomies.
Scholars and policymakers meet with many essential difficulties when they dealt with these complex problems.
This empirical idea is simple, although its theoretical foundation encounters essential difficulties.
As we know, there are many essential difficulties to extend the technique to the degenerate system.
There are three essential difficulties in constructing an arbitrage-free option price function from the available bid-ask quotes.
