I've been doing the campaign under the "Easy" setting and have managed mostly Gold victories (although a couple Bronze as well).
But yeah i agree with you, it's a bit too tough.
King's Bounty: Armoured Princess is also tough tho, as you may know.
A couple things that annoy me about the AI is that they don't care about the towns.
Enemy units will always vacate their defensive position to attack.
Also, i hate that you really have to work hard to finish-off a unit.
They'll run away and heal up next turn.
I've been trying to keep my soft units in the middle, surrounded by defensive Flame Defenders or whatever they are called.
But you have to keep forward progress going so you'll have to expose some archers or whatever from time to time.
It also seems wise to spread your Artifacts out to different units and try to make sure they get used.
But it might be true that it's best to ignore Gold/Silver/Bronze and go for the added XP by hanging round to kill off enemies.
I don't know if other battles become available if you get all Gold or not.
Like, does the campaign path change depending on your victory status of the last battles? (like Panzer General)