There is a simple alternative to spending 1600 MS Points on the whole Metallica pack (or any DLC pack for that matter) just to get the one song for this achievement. You can play One with both players on Expert (most likely can be done with 1 on Expert and the other on Hard).
TIPS:
1) Try to keep a good, long streak going before the alternate-strumming part. If you have around 400,000 points by the time the alt-strumming starts, you're in very good shape.
2) Use Star Power as often as possible, and try to use it as soon as you get it. Having full SP for the alt-strumming part will net you a lot of points. If the lead guitarist thinks they'll have trouble with the triplets solo, then save SP for it. (No point in failing with a minute and a half left.)
3) Watch your score throughout the song. If you can't keep a good streak going during the end solo, then try to have a score of somewhere around 550,000 once the alt-strumming part ends.
**NOTE** This is very important to know, me and Aerodynamo found this out the hard way. If you're playing online, only the HOST will get the achievement. So if both players are trying to get this, you'll have to have 1 person host, get the achievement, switch hosts, and do it again.