Did
Super Best Friends achievement.
Now I'm not sure if it's bugged or not, that's for someone else to determine. All I can say is that it's perfectly solvable.
The strategy?
Add 5 sims to your household in CAS. Recommended popularity asp. and outgoing trait on all.
Now make them BFF with any NPC sims.
Done?
Okay, save the game then go "just manage worlds", kick those 5 dummy sims out into a new (unplayed) household, repeat the process (add 5 sims...).
What happened in my game? I got the achievement on the 10th sim! Not on 20th I've expected.
Now either the requirement is not 20 but 10 (bugged text), or the game also counted BFFs I made during friend of the world aspiration on two sims (that are not BFFs actually any more). On those two sims I really did make 5+5 times BFF with a NPC sim.
Which in the end means someone else will have to determine which case is it. Wrong number in the description or the game adds data from the aspiration.
Pics? Pics.
http://s11.postimg.org/64xg7tp9v/bff1.jpg

http://s3.postimg.org/myo2gddub/bff3.jpg
http://s27.postimg.org/z4zw8iseb/bff10.jpg