I Wore 'Em Down
You must win 50 Attrition matches, joining in the middle of a winning match you do get a win.
The first method I will say is if you want to get this by playing normally and get 50 wins while going for the objective. I have been doing this method for a bit and it seems to be the best if you don't want to join any boosting sessions.
What I did was join a match and if my team was winning I stayed, but once my team loses, I would leave and search again. I have won about 10 in a row with a group because I stayed with people who were really good.
Now once you found a winning game and keep playing until they lose.
This other method can be a bit annoying but it is the fastest especially if you get it going.
Trueachievements has boosting sessions if you go into the boosting session category (or add this game to your boost list) If you host a session, just follow this guide and you will get some fast wins. Create a party and get 6 of the people in it, then create another party and get the other 5 in it, (12 people in a session 3 as back-ups) Have 1 person be the party leader and control that group as you control the other.
Once this is done, both leaders change their servers to JAPAN WEST in the Main menu, then invite everyone in your parties to the games.
When both groups have 6 people in both groups, just search on Attrition at the same time running into each other. Once you find each other and the game starts, the team losing first just back out. Once back on the menu, start searching again to run into the group again and alternate wins.
Any questions or comments? Please comment or send me a message, good luck and have fun!