challenging       
adjective uk/ˈtʃæl.ɪn.dʒɪŋ/ us/ˈtʃæl.ɪn.dʒɪŋ/
B1 有难度的;具有挑战性的;考验人的
difficult, in a way that tests your ability or determination
  This has been a challenging time for us all. 这对我们大家都是一个具有挑战性的时刻。 
 Compare: thought-provoking
 - More examples
 - He resigned from the company in order to take a more challenging job.
 - She doesn't find the general knowledge crossword challenging enough.
 - You did that so well that I'm going to give you something a bit more challenging to do this time.
 - It's when you find yourself in a really challenging situation that you find out what you are capable of.
 - Some of the scenes were extremely challenging for an inexperienced actor.