Make sure you have the highest score when the turn timer runs out, and make sure that when prompted, you select to end the game.
Some game setup that can make this go quicker:
- Only have one other faction. Smack them around to keep them in line if needed, but don't eliminate or ally with them.
- Set Pacing to Fast. You will only have to play through 400 turns then (as opposed to the normal of 800).