I started from the very beginning to the end by myself on realistic and the achievement popped when expected.
No co-op for me. I'm too good for it.
163/164 games completed (including DLC)
Completion percentage: 100%
Latest retail completion: Dragon Age II
Latest arcade completion: Dust: An Elysian Tail
Currently Ranked #1 on Achievement Completion Percentage Leaderboard on TA