I got this achievement as soon I updated gears 4, I was Re-up 11 Level 100 (so just an X button press away from Re-up 12 [Wings 2]) when this popped for me too because I thought it would require you to do the Re-up action. But it didn't require that for me, it was retroactive!
I gained bunches of levels during the 6.66 times XP event during Halloween (130 levels to be exact) and found that Sniper Strike Solo method on Checkout was the fastest way for me to level up at the time (faster than even Speed runs on Inconceivable!).
Sniper Strike method: granted I was doing this on 6.66 times XP so your XP gains will be different from mine but I was averaging 75k xp on a good wave, and 35k xp on a bad wave. Good waves would only be Swarm enemies as you gain xp mostly from the ribbons you earn, and bad waves would be when Sentinels spawn in. Sentinels spawned in about 60% of the time. Checkout was my level I preferred, make sure it's on Inconcievable for maximum XP (adding Iron Man mode on I didn't see any notable increase in XP so I left it off most the time). Remember to start on wave 49. Once you get to the Fabricator pick it up and set it down immediately, this causes the countdown to start so it saves time. Now carry the Fabricator to the Boltok Pistol room near spawn area. Now just spam Sniper Strikes till the wave is completed. On Sentinel rounds try to mark the Sentinel with a Called Shot bonus to kill him quicker, each mark last about 3 Sniper Strikes. Here's what my build was, yours should look as close to that as possible:
Headshot Damage lvl 6
Precision Rifle Damage lvl 6
Sniper Strike lvl 4 minimum, mine was lvl 5
Magic Bullet lvl 4 preferred, mine was lvl 5
Called Shot mine was lvl 6
With the XP event not happening as the time of my writing this then actual organized speedruns are likely faster.
As always, if you have any questions just hit me up and I'll try to answer as best as I can.