"extra challenge" is correct and can be used in written English.
For example, "This extra challenge will help you strengthen your skills."
Yet the LCB faces an extra challenge.
They also received an extra challenge for each tie-break.
The app also has advanced puzzles, which include numbers above 100, for an extra challenge.
But for those whose greatest asset is their business, this recession brings an extra challenge.
But that argument points to the extra challenge their side faces: The opposition is divided on the strike.
Defective parts had been mixed in with new ones thus presenting an extra challenge to the students.
Argentina's proximity to the host nation adds an extra challenge for the security apparatus to deal with.
The extra challenge of the A.P. course may prepare the student better for the challenge of college courses.
Ms. Cooperman faced the extra challenge of a tremor from Parkinson's disease, which causes her knee to shake.
In the film, people's irises are read while they're on the move, presenting the extra challenge of movement and resolution.
An extra challenge was making sure the exhibition had a coherent story that would make sense to the visitor.
