When the AI attacks, they never have a full, organized force. If they are in difficult terrain, they don't even consider special units, like partisans, etc.
This is what I have:
Each army has 4 marines, 2 howitzers, and 2 armor.
First I send in my subs to destory their ships, then I bombard their coastal cities with battleships, then I make an amphibeous assault. Then I isolate and conquer each of their cities.
This is what the AI does:
They send in 2 battleships, which get destroyed by my defending submarines.
Send in 2 transports with varied, ineffective soldiers (e.g. riflemen in the forest, vs. my partisans).