“A pain in the neck” ~ Something bad or inconvenient. Not necessarily a physical “pain”.
“Back to the drawing board” ~ Sometimes you have to start from the beginning, even if you’ve already spent a lot of time working on something.
“Call the shots” ~ If you are the boss then you have the power to make important decisions without having to speak to anyone else.
“Down to the wire” ~ When you don’t have a lot of time to complete a task, you can say that you are “down to the wire”.
“Time to kill” ~ Sometimes you have free time to do whatever you want. For example, maybe you have a meeting at 4pm but arrive at 2pm. You could say that you have 2 hours to kill.
“To rack your brain” ~ Sometimes it’s really difficult to think or remember and to think really intensely, or “rack your brain”.
“Smooth sailing” ~ Sometimes things in life are easy and run without problems. These occasions can be described as “smooth sailing”.