This is not really a solution, but it is an update to the above solution. As of 2/24/18, KillingJammer’s solution works wonderfully.
According to the comments on KillingJammers solution, you must first complete the entire campaign on a lower difficulty, with no skulls activated to initiate this glitch. I played my first campaign playthrough on normal.
Once you’ve done that, start a new campaign via the missions tab. Select Legendary difficulty and then choose your skulls. I am unsure as to if any of the other skulls hinder the glitch in any way. I chose the Scarab, Envy, and Bandana skulls to use during my playthrough.
Once you defeat Tartarus, which is really freaking hard by the way, it will finish with the cut scenes and take you back to the menu. It took roughly one minute for the achievements to pop once I was back to the menu screen. I re-entered into the missions tab and sat there idle, when it finally unlocked.
A bit of advice if you’re stuck on defeating Tartarus as I was for an entire hour. I used the Needler as my primary weapon. When Johnson snipes him three times and when Tartarus’ shield diminishes is when you unload an entire clip of needles into him. Also, throw a human grenade at his feet when you hear Johnson fire his second of three shots. When his third hits, the shield goes away, the grenade explodes just as the shields are down, then empty the clip of needles. Rinse and repeat.
Keep an eye out for his fellow Brutes, dispatch them quickly. After the second round of Brutes is when you know you are extremely close to killing Tartarus.