**Edited Dec. 6 2019 to include map info and clarify which team is defending**
At the time of the MCC: Reach launch, the traditional Invasion mode is located exclusively in a competitive matchmade playlist. It is a 6 v 6 gametype with the one team defending two generators in phase 1 and the other team trying to capture/destroy one of those generators.
To earn the achievement you must win the round in the first phase, which is only possible as the defending team. Two of the maps have the Spartan team defending (Boneyard and Breakpoint) and one map has the Covenant defending (Spire). (The playlist also currently has some Forge World levels, in which either team could be defending.)
If you don't start the game as a defender, don't worry - each team plays one round as the Covenant, and one round as Spartans, switching roles.
Each generator has a 20 second countdown timer associated with it, meaning that the attacking players will have to stand in the small zone surrounding one of the generators for 20 seconds. That time is cumulative, so they can keep coming back to chip away at the timer. Your job as a defender is to keep them off that point so they don't capture/destroy either generator for the entire first phase, which lasts 4 minutes.
The best way to get this is to coordinate with a couple other friends or create a session here to play with some other competent Halo players. It might take a few games, but it isn't too bad. I got lucky and had a good team, in addition to having one of the other team's players quit, which made this pretty simple.