Each week (from Monday to Sunday) the Player vs Player (ranked) stat resets so you start in #15. For each 100 points you will advance a ranking, to 14, 13 and so on until reaching rank 1. You will need 1900 points of experience in PvP to reach that.
When you play on PvP you have three rivals to play, one easy (left), one more or less at your level (middle) and other higher than you (right). Could be a little above you or pretty high (it does not matter the player level or rank, just the sum of the 4 troops you choose to play.
The higher rank you play, the less time you will have to spend advancing in rank but, surely it will be time consuming, and the defeats will decrease your rank number a little bit.
It takes time, if you start at the beggining of the week you will reach #1 before the end surely (playing a bit day by day)https://screenshotscontent-t4002.xboxlive.com/000900000320d2...
BTW: Fixed with March 1st 2017 update since before was not unlocking
BTW2: It looks that the 2nd update from March 1st 2017 got it wrong, because you only have to reach level 2 (1800 xp points) to unlock the achievement. Confirmed by several other gamers and myself.