The achievement description is a bit deceiving. It should read, "while standing within your controlled hill, defend by killing 5 opponents before they enter your hill, in one game. Kills in contested hills do not count." Or something similar...
This can be done in a custom game with a friend over Xbox Live or with a second controller with the following settings:
Go to Multiplayer. Select the following:
1. Custom Games
2. Halo 2 Custom Game3
3. Halo 2: Anniversary Custom Game
4. Choose Map: Warlord (small map)
5. Choose Gametype: King of the Hill, then
6. King (capture point does not move)
a. General Settings:
(1) Respawn Settings:
(I) Respawn time = Instant (to speed up respawn)
b. Time Limit = 18 minutes
c. Score to Win = 5000 (or less, depending on how many people are working on this achievement)
Accept the options and start the game. The ring will be located in the middle of the map on the second level of the tower by the ability spawn.
The player going for the achievement then goes and sits in the ring so that he/she is controlling it. The second player then runs to the ledge looking into the ring BUT DOES NOT ENTER IT! The kills most be against players while it is controlled. Entering the ring changes it to contested, and the kill will not register.
Note: This only worked when there was a time limit and a score limit. Also, THIS MUST BE DONE IN ONE GAME! I confirmed this solution using my secondary account (see Hockyeddie on Xbox Live).
Please add comments if you find a better map or better King of the Hill sub-type, and I will edit my solution.