There are many situations when we want to say we find an activity difficult. It may be a subject we are studying or a skill that is part of a physical activity we are learning.

It's not so easy...
It's a bit tricky...
It's not the easiest ___ in the world...
It's quite tough at times...
It's (quite / a bit) hard going...
It's nigh on impossible...
The course is quite demanding.
The course can be gruelling at times
It isn't a walk in the park
It's not a course for the faint-hearted.

Explanations

• Phrases 1, 2 and 3 are saying that it is a little difficult for some people.
• Phrase 4 says that some parts of the course are difficult.
• Phrase 5 says that the course is difficult and that many people will not keep going and give up before they finish.
• Phrase 6 is saying that the course is nearly impossible.
• Phrases 7 and 8 are saying that you will need a lot of effort and determination to complete the course.
• Phrase 9 is saying that the course is not easy.
• Phrase 10 says that people who give up easily should not start the course as they will probably not finish.