Artisans the hardest one to farm for XD Needs loads of ores LOL
Cooking needs a lot of killing mobs for materials
Card Master needs help from Cooking and Alchemy to make some items, but is relatively easier to level. You only farm for things like Soul Essences, common loot.
Alchemy is also easier to level up and pretty much staple for all characters. Id recommend this one~
Theres an assistant skill guide here
http://noscrubs.net/discussions/guide-assistant-skills-cardmaster-chef-alchemist
It's a bit outdated due to the newer recipes this past updates but still a good source of info to level up