Any suggestions how could I teach my students the two words? I find it quite difficult to make up the exercise for that.
To walk a tightrope is a difficult skill to learn, but if you do so it will be quite an achievement.
Do you find your work to be difficult?
Harry has proven to be a difficult child to teach.
He was very proud of his achievement.
Achievement and success go hand in hand.
Your climbing that mountain was quite an achievement!
I found it so difficult to do that!
Thank you very much for your prompt reply 
I have only joined the forum and find it very interesting.
Wishing you all the best
Great examples, Bill.
I find it difficult/hard to achieve anything. Achievements require a strong focus and a goal-oriented approach. Reaching a goal is a matter of determination and motivation. You need a good incentive to strive and achieve.