"fiercely difficult" is a correct and usable phrase in written English.
Example: The challenge we face is fiercely difficult, but our team is up to the task.
And deflation, once entrenched, is fiercely difficult to dislodge, as Japan has found.
The exception was "Fanfares," Ligeti's relentless and fiercely difficult variant on the two-part invention.
"La Valse" by Ravel also appears in its fiercely difficult reduction from orchestra to solo piano.
Ms. Palmer also came close to mastering the fiercely difficult coloratura of an aria for Cleopatra.
Even with that name on her CV, she and her contemporaries have found it fiercely difficult to get work.
The program included the premiere of a fiercely difficult work by the flinty American composer Frederic Rzewski simply called "Four Hands".
A lonely, difficult, fiercely determined man, Smith needed solitude and direct contact with nature.
And that secret is also, of course, the difficult, fragile, fiercely protected fact of being an artist.
Some important research was done in Britain notably at the University of London's School of Oriental and African Studies (SOAS)—that might have been difficult in the fiercely anti-Communist atmosphere of America in the 1950s and 60s.
Proud, fiercely independent and no doubt difficult in his day, Russell said he didn't need many friends but he had one in Auerbach, right to the end in October 2006 when, he said, Auerbach was still coaching.
The problem is they weren't all that intimidating before Romo was injured; they have other issues besides injuries; they are in a fiercely competitive division with a difficult schedule; and they have a coach, Phillips, who could conceivably be going bye-bye during the break, depending on how irritated Jones was watching his team take such a Giants Stadium beating.
