Solo Co-Op is possible!
The co-op achievements can be granted solo - w/ 2 controllers of course.
Although I finished the campaign years ago, I decided to now play the multi and co-op. Frustrated by not finding people, I used a local co-op with 2 controllers. Place your character in a safe spot and play through with second controller on easy. Be sure to set your Character to insane to get that chevo too. If you have been playing all releases, this should be a cake walk.
Keep track of what chapters you've played with actual people and fill in the gaps with this method. I did this for 13 chapters including all of the last 2 acts.