What idioms can describe a tough or difficult thing (such as a test)?
I am assuming that by "question paper" you mean test.
In that case a tough test might be described:
The test was a [real] bear
From Merriam-Webster:
bear : 4. something difficult to do or deal with- < the oven is a bear to clean >
The test was a ____________!
Nightmare.
2.1 A person or situation that is very difficult to deal with:
OED
A few other nightmarish experiences are applied metaphorically:
Headache:
1.1 informal
A thing or person that causes worry or difficulty; a problem:
OED
Pain in the neck (and a few other less noble parts of the human anatomy)
something or someone that causes trouble;
a source of unhappiness;
Based on WordNet 3.0, Farlex clipart collection. © 2003-2012 Princeton University, Farlex Inc.
thefreedictionary.com
Can of worms
Fig. a very difficult issue or set of problems;
an array of difficulties. (*Typically: be ~; Open ~.)
thefreedictionary.com
Snake pit might have connotations that drag you away from the difficulty of a test but there is surely a metaphorical intersection in the word picture:
any place of pain and turmoil;
thefreedictionary.com
The final two definitely have connotations in another arena, but the word pictures apply metaphorically:
Hornet's nest
A situation that presents difficulty, uncertainty, or perplexity:
American Heritage® Dictionary of the English Language, Fifth Edition. Copyright © 2011 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.
Pandora's box
n.
A source of many unforeseen troubles:
American Heritage® Dictionary of the English Language, Fifth Edition. Copyright © 2011 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.
If you are willing to change the formula, That was a ____________!
Test from hell:
informal
- An extremely unpleasant or troublesome example of something:
OED
The test was a killer.
"Killer" is an informal noun and adjective meaning something that is extremely difficult to deal with.
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