A few random tips...
Nuclear War: if opponent's capital is too far away to nuke, settle a city close enough then rebase the nuke. Become like the annoying AI if need be, that places a city on a 1 tile island. If all the land is taken up, conquer a CS that is close enough.
Sea: get barracks + armory, then take the amphibious promotion so your soldiers don't have to land to attack cities effectively.
More nuclear war: if there is a tough city with lots of defenders, drop the first bomb and then start moving like you are going to take the city with your troops. The opponent should move more troops in thinking the nuking is done, then you drop the second nuke.
Even more nuclear war: apparently nuking a city destroys their nukes. Put your nukes in position so that you can launch all at once, taking out the cities with nukes. You can't always get all their nukes, so just accept that your civ will become happier after they retaliate and reduce the population of your periphery cities.