How to win a Civ V game with super powerful adversary

Solution 1:

Score is mostly an indication of territory, but it doesn't necessarily translate to winning the game. Think about all the different victory conditions possible, and pick one that minimizes Siam's advantage.

• Cultural: This is probably tough to switch to late in the game, but if you're far enough along in the social policy tree, you might be able to push for it.

• Diplomatic: Throw everything towards getting the United Nations, and then save enough money to buy off the city states.

• Science: I'd advise against this one, Siam's territory advantage translates to a big science advantage, you need to win before they finish their spaceship.

• Domination: Remember that you don't need to defeat their entire empire, just take all the capitals. If you can take out your neighbor's capitals, you have a shot at a surgical strike on Siam's capital to win. Don't underestimate the computer's defenses though, they don't go down easily.

Diplomatic might be your best shot, although Siam has likely bought off enough city states to make it difficult. Cultural will only work if you're already well-positioned for it. Otherwise, go for Domination and focus on the capitals.