This can be done solo, if you have a second instrument of the same kind and create a 'dummy' online account (use the 'free trial month offer they give and you'll be able to go online with that account).
I signed in my real account and the dummy account I just created (I gave all fake information when signing up). Then I went to competitive, joined the second account in as the same instrument and difficulty, then selected rockfest and selected find xbox live players. it'll ask you about lowering your difficulty, say yes, but you'll notice now that instead of continuing to search, it will have a count down to the match starting. My first time had the 'do or die' option so I selected that and then the shortest song (like whoa) and at the end of the song both accounts got the achievement.
NOTE - you may not need a second live enabled account, a guest account may work. also, credit for the solution goes to the people that posted a very similar method for the 25 pro face off challenges on your own (which is what I'll be doing next).