750,000 must be achieved solo; co-op scores do not count.
JUST SO WE'RE CLEAR: All Nightmare Long on EXPERT is THE song to get this achievement on unless you are absolutely insane at playing Guitar Hero. See the bottom of the solution for other suggestions at songs that can reach a score of 750K, but 689 out of 1,031 ScoreHero users (67 percent) have gotten 750K with All Nightmare Long. While you cannot buy this song separately as a single, the amount of time that you'd likely spend trying to get 750K on the other songs would likely cost you a lot more time and frustration in the long run.
If you have MS Points to burn, here's the link for the album:http://marketplace.xbox.com/en-US/Product/Metallica-Death-Ma...
As of this writing, 750,000 has not been reached in any song on any difficulty other than Expert.
Although there are several songs listed below in which the 750K mark is possible, only a few are going to be worth trying for most players. Through The Fire and Flames can reach 750K with 94% notes hit and a max note streak of somewhere in the range of 550-800. Since this is the only song on-disc that can reach 750K, it's the one most people are likely going to try the most.
If you are willing to fork over the money for either an XBox Live subscription or downloadable content for your already existing subscription, however, the field changes quite a bit. All Nightmare Long is definitely the easiest of the bunch of downloadable content songs, as it is much more simple than the other 750K songs. While the other songs in the list have complicated riffs, solos, or both, All Nightmare Long is doable with a good understanding of alt-strumming speed and the dexterity to pull off some of the chord changes in the song.
The following songs on Expert have scores over 750,000:
Through The Fire and Flames
Heroes of Our Time, Revolution Deathsquad and Operation Ground and Pound (Dragonforce Pack)
The Devil Went Down To Georgia (FREE)
Cyanide, All Nightmare Long (Metallica Death Magnetic Album)
Ernten Was Wir Säen (DLC Single, not available in a package deal)
The video below is a Full Combo (100% notes hit) run of All Nightmare Long on Expert. The Star Power usage is something to note here, although it may be hard to see at times.