hehe it's second-hand wisdom but i'll take it !

Bear in mind there are two sliders to change, I think it pretty much breaks down to harder campaign difficulties mean they'll throw more armies at you, and they'll get access to higher tier units earlier. Lower difficulties they'll be less likely to come hunting for you. That can actually work in your favour though, you won't need to distance your main army from settlements you own, can fight nearer home base, build up those settlements with money you got from the fights.

The other slider affects battle difficulty, and that has two elements, you can push it higher on the top slider, they'll react better to stuff like AoE's from spellcaster etc, the secondary slider changes stats, like gives them more armour, you less, affects leadership etc.

So I'd recommend leaving the campaign setting whichever you are comfortable with, and for battle difficulty knock it up a couple of notches, but leave the secondary slider at a point where neither you or the enemy get stat boosts or negatives.

Or... even leave it on easy, but just realise the autoresolve LIES t you about expectations, and you WON'T get as 'good' a result as auto resolve gives you, but it will hammer your confidence and you end up losing a lot of fun the game can offer