This achievement is easiest to get as mentioned previously when you rank up off someone who is already a TrueSkill of 50 or in the top 5 on the leaderboards. It should not take very long at all when doing it this way.
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.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 currently hard to get achievement.