I feel like the plains is just right in terms of difficulty, personally, but yes, everyone will have their own idea of how difficult it should be.

Note that the plains much more deadly at night than during the day, because instead of spawning rarely in groups of 1-2, fulings will spawn frequently in groups of 3-5. Unless you're very confident, do yourself a favour and just leave the plains at nightfall.

Also, shamans and berserkers don't spawn randomly, only at a couple of structures (fuling villages, in particular). And although you are pretty much required to attack fuling villages to get the resources needed to upgrade your equipment, you do have the luxury of being able to prepare, choose when you attack, and fight from long range when you do so.