I finally got this achievement a few minutes ago. It took me
six tries. I was very frustrated because beating the final battle 6 times is annoying.
However, since I already got Token Experience, I just lowered the difficulty on this save so it's easy (but still annoying).
Make a save before the final mission when you're in town with Cartman.*** Spoiler - click to reveal ***I made several saves at different times, but saving right before the final battle finally worked. Here is exactly what I did before it popped:
After you teleport back to your bedroom with Cartman, I made a save. At this point, you take a selfie with the burglars. From there you do a timefart that leads you back to town, where you will walk with Cartman for like 10 seconds and then trigger the next cutscene, which is the final fight against Mitch Conner. I did this five times and it did not pop the achievement.
So, I decided to make another save right after you take the selfie with the burglars and you do a timefart teleport. Once you teleport into town post-selfie and you are able to follow Cartman, make a save. Then, trigger the final battle and win. When I loaded this post-selfie save and beat Mitch, the achievement finally %(*#@&! popped.