[Geoff Bacon]

I haven't had the need to do this but the following should work:

• Display "hard" collection
  
• Select all songs (long press a song then press unchecked square at bottom right)
  
• Press Edit at top of screen
  
• Set the difficulty (3rd line from bottom) for each song in this "hard" collection to 5 (say)
  
• Select OK to apply (note the edit should only update fields that have a tick to the left of the name)
  

Repeat for the "easy" collection using a difficulty of 1 (say)
Use different values for any other "playability" named collections.

You can then filter your "Needs more practice" collection using the difficulty field e.g. 5 for hard (this is probably the one you will need most)

Cheers
Geoff