Talisman, if the opportunity is there, that is when you will face an opponent that has a long unbeaten streak, I suspect that the system will favour your side. With a good enough team, the right tactics, even if the opponent is stronger, you will likely to beat him. I have noticed some trends in achievements too when I consider both of my teams.
About the unbeaten achievement, I can't comment much though, but I do have this achievement unlocked. I did it in the lower levels (level 2). Maybe you missed your chance of doing it as for this one, it does not seem likely to occur in the higher levels.