The phrase "a rather difficult task" is correct and usable in written English.
You can use it when describing a task that is challenging or requires significant effort to complete.
Example: "Completing the project on time proved to be a rather difficult task due to unforeseen complications."
Alternatives: "a quite challenging task" or "an especially tough task."
The faculty of Mannes College of Music's annual Beethoven Institute faces a rather difficult task every year in its two faculty concerts.
This makes customization of an efficient search method a rather difficult task.
Defining such connectivity even for a small region of the crystal would be a rather difficult task resulting in a quite complicated and almost useless SMILES string.
This is a rather difficult task in rodents, particularly in mice, in which the required blood volume would exceed the total blood volume of the animal.
The design of n-variable t-resilient functions with strictly almost optimal (SAO) nonlinearity (>2n−1−2n2, n even) appears to be a rather difficult task.
The development of mathematical models for such systems is a rather difficult task, since no equations based on first principles can be written.
Another restriction is that the translation of targets irrespective of their origin into quantitative numbers for 2020, 2030 and 2050 appeared to be not a straightforward, but a complex and rather difficult task, due to several reasons.
Several factors, such as task characteristics, the finding of continuous improvement in performance until the end of the rather long practice session and the presence of non-learners, indicate that the bimanual visuomotor tracking paradigm used in this study is a rather difficult motor task.
It has agreed that something needs to be done, but has put off until another day the rather more difficult task of getting its fractious membership to decide what that should be.
