To get the achievement, you need to complete 50 songs in SCORE ATTACK (any level of difficulty). You will probably be going for EASY, most songs are doable even for less skilled players.
Don't worry, even without DLC, there are plenty of songs to play. And each song can be used THREE TIMES for this achievement. This is how:
There are three paths you can take - lead guitar, rhythm guitar and bass guitar (which you can play even with regular guitar - switch to emulated bass) - for the achievement, a song completed in each of the paths counts - for example if you play Blitzkrieg Bop (the easiest song in the game) in LEAD GUITAR PATH, you get one down. Then you can switch to RHYTHM GUITAR PATH and play it again, another one down. Then switch to bass, another one down.
If you struggle, I suggest switching to bass. The bass line in most songs is very easy on EASY.
You can sort the tracks by difficulty, although... I don't think this really helps much.
The achievement will unlock once you've reached 50 total (across the three paths).
You can get bronze, silver, gold OR platinum, it doesn't matter for this cheevo. If you get three strikes, you fail, obviously.