This achievement can be gotten by playing a few different songs. Most people I know choose "One" on expert (hard may work but I am not entirely sure) because as long as you can keep the note streak up until the darkness riffs and just a little bit into it, then it should not be too difficult. Just make sure the stronger player is on lead. However, there are other options.
The following is a simple list of other options on how to get the achievement:
1) TTFAF on easy (practice because even though it's on easy, overstrums do happen).
2) TTFAF on medium (not entirely difficult, and only suggested for those getting frustrated at getting really far on easy and missing near the end).
3) Story of My life on expert (basically have to full combo since it has only 2024 notes).
4) Even Flow on expert (not too difficult for either lead or bass and is really fun to play. Also with 2121 notes it's pretty much FC or move to another song).
5) Holiday in Cambodia on expert (ignore the intro. This song has plenty of notes afterwords to get that 2000 note streak. In fact, check out www.scorehero.com for co-op videos of this and other songs that you think you may be comfortable with getting that 2000 note streak.
6) There are DLC songs where this is possible as well, but who wants to PAY for an achievement? Well, since I love Metallica, I bought the Death Magnetic Album and my friend and I played Suicide and Redemption K.H. and got the achievement that way (with a killer 3796 note streak btw). Both Suicide and Redemption songs are fairly easy on expert and it can be done as long as the streak is kept until the solo. IMO, J.H. has an easier solo. I am not entirely sure, but this could probably be done on hard as well with both songs.
Well, I hope this helps. There are plenty of options, and if you want to see more songs where this can be done, check out www.scorehero.com. There they have song stats, videos of FC's and CO-OP FC's, top scores, forums, etc.
On a final note, I play GH3 every once and awhile and would not mind helping someone get this achievement if they cannot find a suitable partner to do so. Just message me (or Swift Shadow UK) here or on XBL and I am sure one of us can help get you this achievement.