I got this achievement by myself in a custom game.
In the Multiplayer menu go Halo 2 then select Anniversary. It doesn't matter which map you choose (I chose Stonetown). Then select the normal map variant.
Scroll across to Territories and then select 3 Plots. Accept the options then start.
Once the game starts all you have to do is capture all 3 territories and once you have the last one just wait 30 seconds and the achievement will pop! Happy 10G.