I dunno, for me it was just a matter of playing & playing the songs until I was able to beat them.  I never used practice mode.  But keep in mind I topped out on Hard in the grouping that has the QoTSA song, and never bothered to learn/perfect hammer-on's or pull-off's. 
  
It took me a little while to get out of that 1st grouping on Hard ("Talk Dirty" gave me a ton of problems, too), but I pretty much breezed through the next 2 groupings.