Gears of War 2

King of the Hill is related to annex somewhat. You have to enter the ring that will spawn near a weapon on the map. The point it is at is random. But unlike annex, the location does not change during the round. It will be at the same spot no matter if you get all 120 points out of it at once, or in several tries.

Once you capture the ring for your team, someone on your team MUST stay in the ring, or else it will revert back to neutral. Also, while your team is in possession of the ring, your team-mates will not respawn until you lose possession, giving King of the Hill a big distinction from Annex.

With the latest Gears of War update (update 6), you can get this achievement easily by using "Social Xbox Live". You do not have to do it in a ranked match and with Social Xbox Live, the game will automatically start with Bots if it does not match you to enough or any players. Social King of the Hill takes away the hassle of finding other players who actually play King of the Hill.

Simply select this option in the drop down in Multiplayer and choose King of the Hill in the Game Type drop down. From here start your matchmaking and with any luck you will get Bot players or only 1 or 2 human players. Once the game starts, capture the ring location (area on map you have to defend) and get your teams score up to 120 to win the round. Win two rounds and your team will win the match.

Repeat this three times to get the achievement.

Bots are difficult in Social King of the Hill but might be easier then doing it with expert human players.

NOTE: If playing with other players you will get the full experience that you would in Ranked. If playing only against bots you will only get a certain amount for winning. You ALSO REQUIRE ALL MAP PACKS to play this mode.

In King of The Hill, you will need to control one weapon point on whichever map you play on. This spot will not change until one team gets 120 points for the round. To score points you need a minimum of one person on your team in the hill, wheter he/she is crawling or stunned. As soon as an enemy enters the hill the hill will break and you will need to recapture it by clearing out the enemy. You need to win best out of three, then rinse and repeat until you have done this three times.

Hope this helps!

I am not going to describe the gametype again the other solutions above covered that perfectly.
To win these matches, good teamwork and communication would be great, but if people aren't communicating, then figure out your role on the team. For example, when playing on Gridlock, one round I was the "ring bearer", staying in the ring while the others did everything they could to prevent the other team from getting control. And I of course also protected the ring when threatened. We actually shut them out one round. We fell into roles naturally as per what was needed. If I died, I'd become the defense or the grenade planter, etc.

An update to one of the previous posts: there is now a King of the Hill playlist that you can go to so you don't have to worry about this gametype not being selected by the majority of the players anymore. The players in that playlist for the most part are quite experienced, so you should have no problem getting on a team that works well. I got my 3 wins in 5 games, no problem.