Have your back to the wall  

If you have your back to the wall, you are in serious difficulty.
  "With his back to the wall, the supplier had to accept the deal."




Bite off more than you can chew

  If you bite off more than you can chew, you try to do something
  that is too difficult for you, or more than you can manage.
  "As soon as I started to translate the report, I realized that
  I had bitten off more than I could chew."




Can of worms  

To describe a situation as a can of worms means that it is
  complicated, unpleasant and difficult to deal with.
  "The discovery of the transfer of funds turned out to be a real
  can of worms."



Cross that bridge

when we get to it   This is another way of saying "we will deal with that problem when
  it occurs and not worry about it before".
  "What will happen if we can't repay the loan?"
  "We'll cross that bridge when we come to it."



Do someone's dirty work

.   To do someone's dirty work means to do the unpleasant or
  difficult work that the other person does not want to do.



Get to the bottom of something  

If you get to the bottom of a problem or mystery, you
solve it by
  finding out the true cause of it.
  "We have a problem of goods disappearing during transport.  
  Hopefully the investigation will get to the bottom of it."



Go haywire  

If something goes haywire, it becomes disorganized or goes out of
  control.
  "The photocopier has gone completely haywire. It's only printing half
  of each page!"



Come hell or high water  

To say that you will do do something come hell or high water means
  that you will do it in spite of difficulties involved.
  "Come hell or high water, I've got to be on time for the interview."



High and dry  

If you are left high and dry, you find yourself in a difficult situation
  without help or resources
  "When her husband walked out on her, Amanda was left high and dry
  with two kids to raise."



In a quandary

   If you are in a quandary about something, you find it difficult to decide
  what to do.
  "The job they offer is less interesting but the salary is better. I'm in a
  quandary about what to do."



In over your head  

If you are in over your head, you are involved in something that is
  too difficult for you to handle.
  "I accepted to organize the festival, but I quickly realized that I was in
  over my head."


Let sleeping dogs lie.


   If you tell somebody to let sleeping dogs lie, you are asking them not
   to interfere with a situation because they could cause problems.



A millstone around your neck   


Something described as a millstone around your neck refers to
  a problem or responsibility that becomes a burden and a source of worry.
  " The money he borrowed became a millstone around his neck"




A needle in a haystack


   To refer to something as a needle in a haystack means that it is very
  difficult or impossible to find.
  "Finding a pub in Dublin without knowing its name is like looking for
  a needle in a haystack!"



Other side of the coin

  When you want to mention a different or contradictory aspect of a
  situation, you refer to the other side of the coin.
  "The house is lovely and spacious, but the other side of the coin is
  that it is far from shops and schools."




Paper over the cracks


  To say that someone is papering over the cracks means that they
  are concealing a problem rather than dealing with it effectively.
  "The measures taken to reduce unemployment are just paper over
  the cracks."




In a pickle


   If you are in a pickle, you are in a difficult situation and need help.
  "My car won't start and the trains are on strike today - I'm in a real pickle!"
(no) quick fix   To say that there is no quick fix to a problem means that there is no
  simple solution.
  "There is no quick fix for unemployment;  major reforms are necessary."




Pass the buck


  If you say that someone is passing the buck, you are accusing
  them of not taking responsibility for a problem and expecting
  someone else to handle it.




Put a spanner in the works



  To put a spanner in the works means to cause problems and
  prevent something from happening as planned.
  (In the US, the word 'wrench' or 'monkey wrench' is used instead of
  'spanner')




Scrape the bottom of the barrel


   If you scrape the bottom of the barrel, you use the worst or the
  least satisfactory ideas, things or people because you have no choice.
  "When they started to recruit, they had to scrape the bottom of the
  barrel because the best people had already found jobs elsewhere."
Smooth the path/way   Someone or something that smoothes the path for something,
  makes progress easier or enables it to happen.
  "Speaking the language smoothed the path for negotiations."





On a sticky wicket  



If you find yourself on a sticky wicket, you are in a situation that is
  difficult to deal with.
  "They've refused to sign the contract so we're on a sticky wicket now!"
Take the bull by the horns   To take the bull by the horns means that a person decides to
  act decisively in order to deal with a difficult situation or problem.




Tip of the iceberg  


The tip of the iceberg is the part that is known of a problem or
  situation which is thought to be much more serious.
  "Journalists say that the report on corruption only examines the tip
  of the iceberg."
Upset the applecart   To upset (or to overturn) the applecart means to spoil a satisfactory
  plan or situation
  "I hope Julie doesn't attend the meeting - she could upset the applecart!"





Vote with one's feet

  If you vote with your feet, you show your dislike or disapproval of
  something by leaving.
  "If the conference is boring, people will probably vote with their feet."