Thanks old friend achievement is not missable. Ive completed story mode, side misssion and 100 collectables and I still got it at the end. All I did was:
1. go to batcave and
2. talked to alfred until he repeats
3. then I go into training console then exit (you dont have to do a challange, you just wana go to a loading screen)
4. repeat step 2 - 4
5. the last converstation I had with him was about the joker being in prision and something about visiting (other people had different last conversation). This took me an 1hr cause he keeps repeating himself.
Alfred appears at 3 location (near batplane, near training console, Computer) use detective vision to find him.
After reading comments I believe you need to complete all quest - story, side, wanted quest, collections