Start an Easy 'Hot Seat' game with two players (both on one controller). For this achievement you will have to control 18 of the 36 territories on the board with a score of 1,800. This means that you can only control territories that are worth 100 points. In two player games, the game is won when someone reaches 2,200 points. If you have three or more players, the game will end before you control half the map, so it needs to be two players.
You start off already controlling 9 territories each on the map (all 100 points) in a two player game. You just have to obtain 9 more. Move around the map, taking over 100 point territories, either by conquering Player 2 or by exploring new ones. If you have to land on a 250 or 500 point territory, purposely miss the questions so you don’t take it over. The achievement will unlock when you’ve taken control of the 18th territory.
If you are still in need of Territory achievements, have Player 2 start to take over territories on a particular continent. Player 1 should do this also once you have this achievement (since some territories are over 100 points). If you unlocked four territory achievements in your first ‘Hot-Seat’ game with four players, the other two should come naturally. Just plan it out and you will get them with minimal effort.