Infinite XP
First, successfully complete the game, and unlock the "Death From Above" and "Heavy Takedown" skills in the Tiger path, to easily earn an infinite amount of XP in Pagan Min's fortress located in northern Kyrat. Once the fortress is weakened, sneak through the front doors by using the steps to the left of the main gate and taking down the heavy soldier. Cross the wooden bridge, kill the rooftop soldier, and use the Death From Above takedown on the remaining heavy soldier. Once they have all been defeated, pause the game, and select the "Save" option. Then, allow the remaining snipers and soldiers to kill you. Throw C4, grenades, or fire rockets at your feet to speed up the process. You will respawn right outside the fortress' front doors. Repeat this process as many times as desired to earn an unlimited amount of XP to unlock the remaining skills.