In canon (book 7, after looking for Ravenclaw’s diadem in the Room of Requirements) there is one example of Crabbe-or-Goyle (i.e. a below-average seventh year student) being able to cast fiendfyre. While he could be rather more well-versed than usual in dark spells, this presumably means that fiendfyre is not too hard to cast for experienced fighters, either.
In canon (book 7, after looking for Ravenclaw’s diadem in the Room of Requirements) there is one example of Crabbe-or-Goyle (i.e. a below-average seventh year student) being able to cast fiendfyre. While he could be rather more well-versed than usual in dark spells, this presumably means that fiendfyre is not too hard to cast for experienced fighters, either.
Crabbe, if anybody cares.