In addition to what has already been said, I got this without any difficulty whatsoever on the first solo of "Give It Away" by Red Hot Chili Peppers, which is part of the main setlist for Rock Band Blitz (so no DLC purchase required). The solo is early on in the song (1:45-ish), short, very slow and consists almost entirely of sustained notes. The achievement pops when you finish the solo. There is a second (much harder) solo later on in the song but you only need to do the first one to get the achievement.
If anyone has other suggestions for songs with easy solos (be it from the Blitz setlist or DLC/exports), post them in the comments section and I'll add them to the guide (and credit you).
Alternative suggestions (credits in brackets):
"Moving in Stereo" by The Cars - DLC (lightsup55)
"To Be With You" by Mr. Big - DLC (Mystic Typh00n)