Annex is basically a rendition of King of the Hill. A large circle spawns at random on the map, but always around a spot where you can pick up a map weapon (Torque Bow, Longshot, etc.) The first team to get there and stay there uncontested (without the enemy team entering) for a few seconds will 'cap' the circle.
When a circle is capped, everyone in the circle scores a cap, and the team is awarded one point for every second they remain in control. If the enemy enters the circle, they stop scoring, and if the enemy caps the circle, the enemy starts scoring. Each circle will give 60 total points out before it goes away and reappears elsewhere on the map. The first team to hit the score limit wins the round.
Play 100 matches of Annex with 3 or more rounds, and get 3 or more caps in every match to get this achievement.
None of the 250 expansion pack achievements have to be done in ranked. They can be done in player, ranked, or a mix of both.
These can also be done with a two-controller method on a private player match. Simply log in two controllers as Gamertag and Gamertag, put them on either team, and make sure your main account wins or does whatever the achievement requires him to do.
3+ rounds means that the win limit must be 3 rounds or higher. For example, if a game that is set at 2 rounds is won 2-1, although 3 rounds were played, it will not count.