This is for getting to rank 14 "Camandante". The fastest way to rank up is by completing missions and Yaran Stories. You also get xp for liberating bases, capturing checkpoints, completing ambushes, stealing supply drops, and finding collectibles (just to name a few). You should have no trouble getting this before the end of the game. The ranks are as follows:
1 Machetero
2 Guerillero
3 Explorador
4 Espia
5 Cabo
6 Sargento
7 Teniente
8 Capitan
9 Mayor
10 Coronel
11 Mando de Columna
12 General
13 Subcomandante
14 Comandante