- It’s going to be difficult and time consuming – I really have a mountain to climb.
- I have a lot of work to do – I have a mountain of work.
- Don’t make a mountain out of a molehill / don’t make a big deal out of sth which is not so important.
- I would do anything to be with you – I would move mountains to be with you.
- When a person is past their prime or best years, we can say ‘He is over the hill’.
- When a situation gets worse and worse we can say ‘It’s all downhill from here’.
English (UK) 
Polski