All skills except maybe theiving can be bought in their own way. mining/fishing ect. you can hire runners when you are powering that trust you, and are fast at trading. Agility is probably the second least, but their is super energy pots which can help a bit if you have time to run and drink. Has to be Theiving and agility for the fact they don't provide you with much rewards, besides the energy restoration fact. But it seems that hunter takes the longest to level.