Updating this now that I've gotten the achievement.
As an alternative/addition to the "play the game a whole bunch" solutions, it is possible to somewhat idle boost this by exploiting talky songs. Talky songs are those ones where there is no pitch on vocals so you can hold your mic up to your speaker, let the song play & get 100% on brutal difficulty (XP rates are the highest on brutal).
Personally I found it worked better when I played pink noise (see note at end for link to YT video of pink noise) through my speakers & put my mic up to that, but some songs work well enough if you just have the in-game audio going through your speakers, and that has the advantages of A) you get to listen to music while you do other stuff and B) you know exactly when the song ends & you need to press the A button without watching/looking at the screen.
To make things go a bit smoothly I typically would create a setlist of talky songs and play that setlist. Note that once you start the setlist the only button you have to press to play the next song is the A button, so if you have a wired turbo controller you could probably just set it up to mash the A button, walk away, and leave it running while you go to work, sleep, etc. I don't have a turbo controller so I can't verify this.
Also note that as of the November 2016 update, XP rate is prorated to the length of song, so song length is no longer really a factor, however, technically longer songs are slightly more favorable when you factor in load times. With this in mind, probably playing "Dead Black (Heart of Ice)" over and over is the best, but unless you want to repeatedly listen to that song it's probably not worth worrying about song length.
As an addition/alternative/compliment to this approach, you can also take advantage of the 1st/2nd play bonuses you get for playing songs in the weekly challenge. Basically enter the Rivals menu, hit RB to go to play the weekly challenge and you'll see a list of all songs you have that are part of the challenge theme. Make a setlist of all of them, and play it. If you have No Fail mode turned on, you'll complete the song (0 stars, but still complete it), so you'll get the 1st play XP bonus (which varies depending on song length). Then do the setlist again and you'll get the 2nd play bonus. Having said that, this is going to be a much lower XP rate than talky songs, but has the upside of the fact you don't have to listen to the same set of talky songs over and over (ie it's a bit of variety).
"Talky" Songs I'm aware of:
Tame - Pixies
Thank You Boys - Jane's Addiction
Testify - Rage Against the Machine
Sabotage - Beastie Boys
So What'cha Want - Beastie Boys
Give It Away - Red Hot Chili Peppers (note that this isn't a FC as there's a singy part at the end)
Debaser - Pixies
Bulls on Parade - Rage Against the Machine
Bombtrack - Rage Against the Machine
Guerrilla Radio - Rage Against the Machine
Killing in the Name of - Rage Against the Machine
Sleep Now in the Fire - Rage Against the Machine
Blinded by Fear - At the Gate
Thrasher - Evile
I'm the Man - Anthrax
The Underground in America - Pantera
(Reprise) Sandblasted Skin - Pantera
The Great Southern Trendkill - Pantera
Dawn Patrol - Megadeth
Laid to Rest - Lamb of God
D.O.A. - The Haunted
This is Exile - Whitechapel
Conquer All - Bohemoth
Embedded - Job for a Cowboy
Aesthetics of Hate - Machine Head
Clouds over California - DevilDriver
Dead Black (Heart of Ice) - Soul Remnants
Got Your Six - Five Finger Death Punch
"Raining Blood" - Slayer (thanks otaku1412)
"Visions" - Abnormality (thanks otaku1412)
Black Dahlia Murder (thanks otaku1412)
For context on how long this achievement takes: level 50 is 160,000 XP, and a FC on brutal of Dead Black gives you 535 XP and is about 8:30 long, so that's roughly:
160,000 / 535 ~= 299 plays
299 plays * 8.5 mins per play ~= 2541.5 minutes
2541.5 minutes / 60 mins per hour ~= ballpark 42 hours to go from 0 to 50
If you play the weekly challenges in rivals mode and take advantage of the 1st/2nd play bonuses that helps (so long as you can still play on expert & GS the song). You also get a small XP bonus at the end of every weekly challenge for "loyalty bonus" if you remain in the same Rivals crew (last one I got was about 1,000 XP).
Youtube video of 10 hours of pink noise:
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