The phrase "an extremely difficult challenge" is correct and usable in written English.
You can use it when describing a task or situation that is particularly hard to overcome or accomplish.
Example: "Completing the marathon was an extremely difficult challenge, but I felt a great sense of achievement afterward."
Alternatives: "a very tough challenge" or "a highly demanding challenge."
The report also said that the issue of personnel would present an "extremely difficult challenge to overcome".
Therefore, it still remains an extremely difficult challenge to find a simple and mild synthetic route to synthesize well-controlled ZnO nanoparticles.
Looking ahead at our mission of accelerating the advent of sustainable transport and energy, which is important for all life on Earth, we face an extremely difficult challenge: making our cars, batteries and solar products cost-competitive with fossil fuels.
Ohio Democratic political consultant Greg Haas said Democrats face an extremely difficult challenge if their goal is to gain control of the state's legislature.
An attempt to recreate a human kidney in vitro by means of a scaffold constitutes an extremely difficult challenge.
Regarding the number of AACs in the stratum oriens, please note that AACs are one of the rarest cell type in the adult hippocampus (Bezaire & Soltesz, 2013; Baude et al., 2007), therefore, increasing the number of successfully recorded, visualized and immunocytochemically or electron microscopically identified AACs is an extremely difficult challenge.
The president himself may have finessed his way through another extremely difficult challenge to his authority, but he is likely to be seen as weakened by concessions to the Houthis extracted by force.
Central to the field of critical care neurology is the extremely difficult challenge for practitioners not only to prognosticate but to prognosticate well.
The circuit design task poses an extremely difficult intellectual challenge.
"It's such an obvious idea, but it's an extremely difficult technical challenge," he said.
The collection of data in the field can be an extremely difficult logistical challenge.
