Hey all, For this achievement as it reads you need to collect 15 UNIQUE warrior treasures. The earliest you can get this is in Mission 5, if you've collected all the warrior treasures thus far. Here is a playlist showing how to get all of the treasures so you can grab this one. Hope it helps!
https://www.youtube.com/playlist?list=PLVZY3gy6E02K5woa-nQnE...