If you're looking for a good way to farm easy XP/salvage and have some patience - there's a small area in Shanty town with unlimited chickens and boars that constantly respawn without you having to leave the area.
You don't get much XP from them but as they are so numerous it's easy to rack up the XP for the last few upgrades you need (I needed 4 for my shotgun, got more than enough XP from this area in about 20 minutes)
Just kill everything and keep running around the shack in the middle and across the river bridge, you'll never run out of things to kill!
There's also an ammo crate at the top of the steps (just to the right of Lara in the above screenshot) that will respawn often too, in case you run out of ammo.
UPDATE from Mainstream:Fastest and most effective way to acquire XP after completing the story:
Visit the Coastal Forest region of the map after you've completed the story.
There will be an indefinite amount of deer wandering around in the area and they will re-spawn each time you kill one. Each deer that you kill and loot will earn you 75 experience and 12-15 salvage. The amount will not change no matter how many you kill and loot, so feel free to use it as much as you need in order to purchase the skills and unlock the achievement.
If you farm here before completing the story, there will be a hunting limit enabled, meaning you will not be able to receive as much experience. This is pretty much only for those that have completed the story and are lacking enough experience to purchase all the skills for the achievement.