Far Cry® 3

The amount of EXP is advisable to obtain by completing mission stories. After you are done with the story if you still lack EXP you can do the following:

• Relic - 250 EXP
  Outpost:
• 500 EXP when detected and enemies are alarmed.
• 550 EXP when detected but alarms are off.
• 1500 EXP when 100% undetected and alarms not set.

The way you can also grind EXP is by completing the game story and reset Outpost, this will be handy when you lack more EXP but I advise hunting relics for an achievement which is reaching 60 relics as well at least you will do two birds with one stone.

There are 34 outposts to capture and claim, at first you will claim outposts even the easiest ones for example at the Northern Island without being detected which you should do for more EXP but it is fine to go loud and guns blazing. Later on, you will acquire powerful firearms to combat enemies especially snipers and heavies with armor (which you should either use flamethrower or explosives like an RPG-7 or a GL-94 though I would advise a GL-94 on the heavies) when you go to the Southern parts of the island. It is best to do them during or after story so you can use them when collecting items and collectables.