The ia is not that challenging, its just the Italian campaign that is the hardest of all because the little room you have to make mistakes. And the massive cheats the ai gets on very hard/very hard.
For that campaign, attack with the guy in north the city and leave him there. Napoleon to the east immediately, capture on the same turn the city and the next turn Austria will attack you on favourable terms for you. Then north to north, sack that province an leave it to revolt. From there just go to Milan, those cities will be undefended and you will be able to take all Italian Austria without shooting a single bullet. Once you do Venice will declare war on you, go straight for their capital. And you have free road to take the true objective. There will be likely 1 full army blocking your way, and a full army ready to counter attack you on the capital after that.
Still the AI is BAD
Tactical AI: make a single line with all units, if thinks can win will send the line to attack without much concert. Also will rotate the line, to match your deployment, but leaving guns behind.
Strategic AI: defend only the city in the capital of nation, attack with anything that is not in capital. This is specially noticeable on Egypt campaign, where the "forts" will be completely undefended always.
Difficulties, the hardest levels only change how much the IA cheats,