Halo: Reach

As is obvious from looking at the achievement description, this requires the defending team to win a game of invasion (not invasion slayer), in the first phase (ie: before the first of 2 capture points have been taken over by the attackers.) In my opinion and experience, the best map to try and get this is on Boneyard. You can have a couple of people looking down at the spawn area, and a couple of people on each side watching the stairways for attackers.

There are basically 3 ways to get this achievement:

1) Play allot of invasion, hope your team works together and defend the best you can.
2) Same as above, but hope some people on the other team quit (for what ever reason), leaving your team with the advantage in player numbers. Thus making it easier to win, and if that happens to be on Boneyard, it's an extra bonus.
3) Get 10-12 people together and boost it. Simply get all the players in a party and search invasion. The attackers agree not to take over either of the capture points, and that's it. Done.

I was lucky to get it with method 2 where 3 people on the other team quit in a Boneyard invasion match. We had everyone split up at the stairways so it was easy to defend.

Good luck :)

To win a game of Invasion in the 1st phase your team must not allow any locations to be taken while your team is defending. Essentially you must 'shutout' the other team. It does not matter if you are attacking or defending first. Just don't let them take a location when they're attacking!

Regardless of map, I would recommend using the Needle Rifle and have 4-5 members of your team rush the enemy spawn. Make sure to leave 1-2 a bit closer to the territories in case someone gets through. If you manage to kill the attacking team off the bat, camping their spawn points is very effective! You'll have 5 of your players simultaneously shooting 1-2 of the other teams players! You can see how that's a recipe for disaster for the other team. You'll need to hold them off for 4 minutes. Make sure to use good teamwork and call out enemy positions!

Alternately this can be boosted by taking 12 people (guests allowed) into matchmaking. Sometimes skill will not be enough and you have to have a bit of luck play into it. I actually got this achievement after 4 players on the other team quit, leaving my team with a 6 on 2 advantage.

I should also note that you cannot get this achievement by playing Invasion Slayer.

This is not quite an entire new solution but a little tip that helped me finally get it.

Do not be afraid to die.

I explain myself:

After getting just about every other achievement in the game I was left with 4, two of them are just a matter of time, Captain and Lt Colonel purchase, one was a matter of skill, skunked, and the last one seems to be a matter of pure dumb luck, If they came to hear me beg.

I've been playing invasion every night for the last 2 weeks or so with very frustrating results because as many have pointed out some people will vote for invasion slayer or not have commo or your team will suck etc. After deciding I would never get it I took one last chance at it last night and, lo and behold, it popped.

So finally for the tip, DO NOT WORRY ABOUT DYING, but make your deaths count, when you are defending use your sticky grenades at point blank range, even if it means you die with the enemy; you will spawn close to the defending point while they go back all the way to the river (Spire). That's how I got mine, granted I had a good team in place finally but it was me dying while sticking the enemy about four times that kept my undermanned defending point safe because everybody else save one other guy was tending to the other point.

Sorry, long rant about something so simple but it helped me, hope it helps you too.