No quick fix here. Everything you do in tomb raider lands you xp. Hunting, Killing, head shots, looting bodies, collectibles and challenges.
It is possible to get it all in one play if you do all of the above the best you can. Every full ex meter is worth 1 skill point which you can use to upgrade skills at any camp site.
If your just shy 1 skill point after you've finished the game re-visit areas and hunt animals