I personally found none of the Advance War games ever difficult, or Fire Emblem for that matter until Hard Mode, even then most of the missions were still easy if not a bit drawn out.
Granted, the missions scale up to unfair when the enemy has death rays, infinitely producing factories of units, and a stupidly large army, but the missions are often set up in such a way that there are key points in the battle field that if you grab, the battle is won. Take AW2 Orange Star final mission, that bridge north of base is key. Grab that and you win.
The AI is also pretty predictable.
Human players, on the other hand, I can't seem to win against. <.<;
Regardless, I sold my copy of AWDR, so I can't join.