I did this for ALL of the 100x weapon achievements, as well as the achievement for 100x revives, and you can do all of them in no more than 200 matches.
For this method, you will need at least 2GB of RAM on your computer. 4GB is preferred, though.
You will also need a minimum of THREE (3) gamertags - Your main gamertag and two 'boosting' accounts. NONE of these gamertags have to be gold Xbox LIVE members, as on the PC, and Games For Windows LIVE, you can play online without being a gold member. You also need to be using Windows XP, Windows Vista or Windows 7.
Next thing you want is to have three seperate IDs on your computer. This can be your main ID on your computer, and then if you're the administrator, add two more, without passwords.
Next, sign onto Gears PC on your MAIN computer ID as your main gamertag.
On this gamertag, load up a PRIVATE PLAYER MATCH. Set it to 3 rounds, with a round duration of 9 minutes. Set the map to Gridlock, as I believe this is the quickest map to use. Also set Friendly Fire to "Yes".
With the weapons unswapped, there will be a Longshot (Sniper) up the stairs in the middle of the map. Just in front of the Longshot (Sniper), in the middle of two sets of rusted cars, there is a set of Frag Grenades.
Next, add the two gamertags that are your boosting gamertags to your friendlist on your main gamertag. Then send them game invites. You can still do this with them showing offline.
The game lobby will load. Set your main gamertag to be on the LOCUST team. The reason for this is that if one of your boosting ideas gets kicked by the connection, you can re-invite them, and they'll automatically join the COG team when they come back. Obviously, you'll have to alt-tab, switch user, and accept the invite again. Now, press Alt-Tab to go to your desktop (Assuming you're playing on windows XP, Vista or 7). This will minimize Gears PC to the taskbar.
Now, use Windows XP's/Vista's/7's ability to Switch User. Load up the second ID on your computer. Load up Gears PC. If this is the first time you're logging onto Gears PC on this computer ID, it may ask for your Gears PC's serial code. You can use it again, even if you've used it for your main gamertag, so don't worry about it. Once that's inputted, and you've signed on to gears PC under computer ID #2, you'll want to accept the game invite from your main gamertag. When you arrive in the game, move your first boosting gamertag to the COG team.
Alt-tab to the computer desktop so that it minimizes the Gears PC window for Boosting Gamertag #1.
Repeat the above paragraph for Computer ID #3 and Boosting Gamertag #2. Set Boosting Gamertag #2 to be on the COG team.
Now, alt-tab to desktop to minimize Gears PC again, and switch back to your MAIN computer ID with your MAIN gamertag.
Your computer should now be doing this:
Main ID: Main Gamertag (LOCUST)
Comp ID #2: Boosting Gamertag #1 (COG)
Comp ID #3: Boosting Gamertag #2 (COG)
With you now on your main computer ID and on your main gamertag, as your main gamertag is hosting, you DO NOT need to switch back to your boosting gamertags for 100 matches, assuming the connection stays stable, as in player matches you do not go back to the main menu after the game, you go back to the game lobby. As the host, you can press Start (if using a wired Xbox 360 USB gamepad), or whatever the start button is for the Keyboard control system. This will force ALL players in the game to ready up - including your boosting accounts!.
With two players on the COG team, and you on the Locust, this means you can go for SIX weapon achievements in 100 matches. Please note that you CANNOT get multiple kills with one weapon in a game. Unlike the Xbox 360 version of Gears of War, you have to get a kill in 100 different matches, not 100 kills.
For the first hundred matches, I recommend getting kills in the following way:
Round 1: Longshot (WITH A HEADSHOT - this counts towards "The Money Shot" achievement) & Pistol. You'll be using the Snub Pistol. This is the pistol you spawn with. With the pistol, a full clip will down the opponent. Reload, and kill him whilst he's down. Make sure you do not get a headshot when the player is down, as it will not count as a pistol kill.
Round 2: Chainsaw & Grenade Tag. You must MELEE-ATTACH the Frag Grenade to your boosting ID for it to count. Simply throwing the grenades and having them die from it WILL NEVER COUNT. I found this out the hard way.
Round 3: Curb Stomp & Execution. With your shotgun equipped (the one you spawn with), melee (B on 360 Pad) each opponent twice. Press X (360 Pad) to do a curb stomp on one of them, and B to do a melee whilst they're down. This will count as an execution.
Each match, if you've got a fast computer, should take between 2 minutes & 35 seconds to 2 minutes and 50 seconds. This means you can get seven achievements WITHIN FIVE HOURS of boosting by yourself.
The achievements that should pop are the following:
The Pistolero achievement in Gears of War (GFWL) worth 94 pointsKill an enemy with a pistol in 100 multiplayer (Player or Ranked) matches of 3+ rounds The Curb Appeal achievement in Gears of War (GFWL) worth 90 pointsKill an enemy with a curb stomp in 100 multiplayer (Player or Ranked) matches of 3+ rounds
You will now have to leave the game lobby as your main gamertag. You will need to make a new private game. Your previous settings should have saved. Press X (360 Pad) to go to the weapon-swap options. Switch the Frag Grenades for the Hammer of Dawn, and switch the Longshot (Sniper) for Torque Bow.
Make the lobby, and do what you did to make the lobby last time, with all the IDs. Except this time, have one of your boosting accounts on the LOCUST team with you. As soon as Round 1 starts, down the boosting account on your team, and then revive him. This will count towards the Revives achievement.
With the weapons swapped, you can now get the Torque Bow and Hammer of Dawn kills in Rounds 1 and 2. Round 3 is where you need to do Boomshot, which is in it's default spawn, opposite the Longshot's (Sniper) default spawn. After 100 games of doing this, you'll get the following achievements:
The Don't Hurt 'Em achievement in Gears of War (GFWL) worth 98 pointsKill an enemy with the Hammer of Dawn in 100 multiplayer (Player or Ranked) matches of 3+ rounds The The Nuge achievement in Gears of War (GFWL) worth 93 pointsKill an enemy with the Torque Bow in 100 multiplayer (Player or Ranked) matches of 3+ rounds
I understand this guide was long, but I thought it warranted it, and I hope it helps those who find boosting with others on this difficult. This is also quicker than boosting with someone else because you don't have to do 2x the amount of matches so that they can get the kills too. Good luck!