I'm using five words to describe the difficulty levels of some training material:
'Easy', 'Normal', 'Hard', 'Challenging', 'Level 5'
'Level 5' is the most difficult level. I haven't found a proper word for that. Anyway, I first want to know some better words to name the five levels. The words are not necessarily related to any professional field because it contains training material from different professional fields.
If no better words are available, please help me with a word 'Level 5'.