If they are in difficult terrain, they don't even consider special units, like partisans, etc. 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. THey build cities that overlap with their radi. Will not kill diplomat sitting outside their city if no 'attack' units present.
Alpine troops are pretty good.
Really annoying when you have to wipe out a perfectly good city just to rebuild it one or two squares away. All you do is spend just a bit longer building your own and it gets there in only five years. A barbarian ship wants to land troops to attack my city. I put two settlers there to block the shoreline. That ship would have waited there forever.