This achievement is easiest to get when you rank up off someone who is already a TrueSkill of 50. It should not take very long at all when doing it this way; probably around 7-9 matches for those with a clean slate.
You need to have 0 losses for it to work effectively otherwise a lot more wins are required. If you have just 1 loss you will need to be boosted up the leaderboards gradually using other lower TrueSkill rank players so as not to waste the effectiveness of the TrueSkill 50 gamers.
Just to note, you can have the person losing quit out of the matches and it will still count. This will make it a lot quicker.
Finally, the achievement only pops when you check the leaderboard so do that after your 7th match (or higher).There is a link here to a pass the parcel thread I set up to help you find who you can get the achievement from. This should hopefully be your gateway to the achievement!
I hope that this helps you get this elusive and frustrating achievement.