Câu đố này hóc búa.

Yes. Đố by itself is connected to challenging someone, quizzing someone, or giving a riddle. But câu đố is the normal noun meaning riddle.

Here, câu helps form a noun phrase referring to a single verbal item, like an utterance or question. So learners should usually remember câu đố as one set expression meaning riddle, rather than trying to translate each part separately every time.

In Vietnamese, demonstratives like này and đó usually come after the noun phrase, not before it.

So:

• câu đố này = this riddle
• cuốn sách này = this book
• người này = this person

That is one of the big word-order differences from English.

Vietnamese often does not use a copula like is before adjectives.

So:

• Câu đố này hóc búa. = This riddle is tricky.

This structure is very normal:
subject + adjective

You usually use là to connect one noun phrase to another noun phrase, not a noun to an adjective.

For example:

• Đây là câu đố này. would mean something like This is this riddle.
• But with an adjective, you normally say Câu đố này hóc búa, not Câu đố này là hóc búa.

Hóc búa is an adjective. It means something like tricky, thorny, tough, or hard to solve.

It is often used for things like:

• questions
• riddles
• problems
• cases
• issues

So in this sentence, it describes the riddle as difficult in a challenging, brain-teasing way.

Khó is the general word for difficult.
Hóc búa is more specific and colorful. It suggests something is not just hard, but especially tricky, knotty, or challenging to figure out.

So:

• Câu đố này khó. = This riddle is difficult.
• Câu đố này hóc búa. = This riddle is tricky / really challenging.

For a riddle, hóc búa often feels especially appropriate because riddles are meant to make you think.

In Câu đố này hóc búa, hóc búa is the predicate adjective. It tells you what the subject is like.

So the structure is:

• Câu đố này = subject
• hóc búa = predicate adjective

That is different from:

• câu đố hóc búa này

That phrase means this tricky riddle, where hóc búa is inside the noun phrase and directly modifies câu đố.

So compare:

• Câu đố này hóc búa. = This riddle is tricky.
• Câu đố hóc búa này... = This tricky riddle...

Yes. Degree words work very naturally with hóc búa.

For example:

• Câu đố này rất hóc búa. = This riddle is very tricky.
• Câu đố này khá hóc búa. = This riddle is fairly tricky.
• Câu đố này hóc búa quá. = This riddle is too / so tricky.

This is a very common pattern in Vietnamese: subject + degree word + adjective

Usually, hóc búa is used for problems, questions, tasks, situations, or issues rather than for people.

Natural uses include things like:

• câu hỏi hóc búa = a tricky question
• vấn đề hóc búa = a thorny problem
• ca khó, hóc búa = a difficult case

For people, Vietnamese would more often use other adjectives depending on the meaning, such as:

• khó hiểu = hard to understand
• khó tính = hard to please / picky
• phức tạp = complicated

So it is best to think of hóc búa as describing a challenge, not a person.

A useful tone breakdown is:

• câu — level tone
• đố — rising tone
• này — falling tone
• hóc — rising tone
• búa — rising tone

A rough English-style guide would be something like:

• câu ≈ a bit like goh with a Vietnamese vowel
• đố ≈ doh with a rising tone
• này ≈ nye with a falling tone
• hóc ≈ hawk with a rising tone
• búa ≈ boo-uh with a rising tone

But the tones and vowels matter a lot, so it is best to listen to native audio if possible.

Yes. It is short, natural, and completely normal Vietnamese.

It sounds like something a native speaker might say when reacting to a puzzle, quiz question, or brain teaser. Hóc búa is a good choice because it sounds a bit more vivid than just khó.

So this sentence is both grammatical and natural: Câu đố này hóc búa.