The phrase "challenging task" is correct and can be used in written English. You can use it when referring to something that is difficult to complete or accomplish. For example, "Tackling climate change will be a challenging task, but one that must be accomplished to protect our environment.".
Defining nonfictional prose literature is an immensely challenging task.
The valuation of patents is an important, albeit challenging task.
New Autry chief executive saddles up for challenging task.
New Autry chief saddles up for challenging task.
However, discovering novel SNPs is a scientifically challenging task.
It is a challenging task.
Local authorities face a challenging task.
Awesome tool! I started using it one year ago and I never had to look for another app
Ha Thuy Vy
MA of Applied Linguistic, Maquarie University, Australia