The phrase "a more arduous task" is correct and usable in written English.
It can be used when comparing the difficulty of tasks, indicating that one task is more challenging than another.
Example: "Completing the project on time proved to be a more arduous task than we initially anticipated."
Alternatives: "a tougher challenge" or "a more difficult undertaking".
Clemenceau found that building the peace was a more arduous task than winning the war.
The wave of passion and excitement that coursed through Mr. Obama's headquarters here in 2008 has been replaced with a methodical and workmanlike approach to manufacturing the winning coalition that came together more organically and enthusiastically for him the last time, a more arduous task with no guarantee of success.
In certain cases the process was greatly facilitated by the above mentioned NICE [3] and ACCP [1] guidelines, whereas in others it was a more arduous task.
Compound that with the fact she's never played competitively with her teammates, and it seems like a more arduous task.
Finding Mr. Zopf a second time proved to be a more arduous task than the first time around.
In villages where conflict has made it harder for basic health services to reach children, community health workers face a more arduous task.
After saving the city's team because he had fond memories of watching University of Washington college gridiron games in Seattle with his father, a far more arduous task for Allen was trying to drag it out of the mediocrity which hallmarked its first two decades in the league.
This was a much more arduous task than writing the expedition narrative.
Unlike Dominoes, bad cocaine, or the other vices in my life, purchasing pills seems like an altogether more arduous task.
They now face the more arduous task of agreeing on which tax deductions to take away, a process sure to pit party members against each other and put them under extreme pressure from outside lobby groups fighting to protect their favored tax breaks.
And while his loss of vision was certainly debilitating, the more arduous task for Nguyen was swallowing his pride and asking for help.
