Skirmishes are probably the easiest, considering they don't have any special rules or conditions. Before the "new" client, there were very easy (and fun-to-farm) Campaigns, but they refuse to reinstate them for some reason.

So they're all painful?

No, but there's a learning curve for some/most. If you know what to expect, they can be easy.
Sometimes you need to protect something, or use alternative damage types, or enemies will (re)spawn all around the map, etc.

like Xiere said "Skirmishes" is nearly the best way to learn the basics - they are not too hard -
also the Training Quest to get the first decks/cards.
you can play the Quests , "classic, nora wars, campaigns, maljaran" and so on too.
on diificult normal they are pretty easy - maybe also a way to learn and earn some Gold too.

finally , if you play these quests it is recommended to play all of these Quests on difficult "legendary"(or the hardest one not sure about the correct word) - with some luck - and if you complete a full "Questline" on hardest difficult an limited card/rune can be the reward -- for Collectors ;)

There are only two-or-three campaigns that have a legendary difficulty reward, and they will specify in their description. All campaigns used to all give better rewards on higher difficulties, but now it's the same base rewards on normal, hard or legendary.

oh boy, this learn curve is hard. I remember playing the campaigns and suffering in some so much, so fun
I played until I got a bug that happened everytime in one quest, was in the end of a human village or dwarf village, don't remeber, but was so frustrating with all the efforts