They're <devilish hard> to capture

[park sang joon]

A sorcerer and one of royal family member of Courts of Chaos, the protagonist got into his best friend Luke's dream about "Alice's Adventures in Wonderland."
He stepped down into the white rabbit hole, carrying Luke fainted, to evade a invincible Fire Angel, it chased after them, and a Jabberwock went after it.
They all fell through the tunnel and the two beasts landed through the opening following the protagonist and Luke.

There was no reason for a Fire Angel to be stalking about this far from Chaos unless it had been sent. They're devilish hard to capture, harder to train, and dangerous to handle.
["Blood of Amber" of The Great Book of Amber by Roger Zelazny]
I know there is a case the former plays the role of an adverb when two adjective is used sequentially.
And I'd like to know if an adjective "devilish" adverbially modifies "hard."
Thank you in advance for your help.

 

[Schimmelreiter]

devilish can be an adverb, and is in this case, meaning extremely.

 

[perpend]

I agree with SR.

In American English, I'd prefer:
they are a devil to capture
they are devilishly hard to capture

This writer doesn't write conventionally (Zelazny), but you know that PSJ.

 

[park sang joon]

Thank you, Schimmelreiter and perpend, for your very helpful answer.

 

[Glenfarclas]

"Devilish" is principally an adjective, but the Oxford English Dictionary lists it as an adverb too: "adv. = devilishly 2; excessively, exceedingly, enormously: originally of things bad, but in later use a mere coarse intensive."

 

[park sang joon]

Thank you, Glenfarclas, for your further information.