- Estimated achievement difficulty: 8/10
- Offline: 31/57 
- Online: 26/57 
- Approximate amount of time to 1250: 400+ hours [legit] 150+ hours [boosted]
- Minimum number of playthroughs needed: 2 [Casual/Hardcore & Insane]
- Number of missable achievements: None
- Do cheat codes disable achievements?: No Cheats
- Does difficulty affect achievements?: Yes
- Glitchy achievements: All weapon achievements & "Seriously..." do not unlock exactly after "X" number of kills.
- Unobtainable achievements: None
- Extra equipment needed?: None
Gears of War is probably best known for its multiplayer, and also known for its infamous "Seriously.." achievement. That achievement is known for not unlocking at 10,000 kills, as most people have it unlocked at 13,000+. The same problem applies to the weapon achievements such as "The Nuge" and "The Money Shot". For these I believe they should start unlocking around the 130+ range. Single Player-wise
Step 1: Playthrough on Casual
The campaign in Gears of War is very straightforward. For this playthrough I recommend doing it in co-op, and on Casual. During this campaign I’d also go for all the Cog tags rather then doing them on Insane. Just use the Cog Tag Locations Guide to help you as it will come in handy. You will also want to focus on getting good for at reloading, not only for the achievements, but for making the game easier.
Step 2: Playthrough on Insane
First of all, for this playthrough make sure you and your partner switch places as Dom to make sure you both get the Co-op achievements. Now simply play through the campaign one more time on this harder difficulty setting. As you unlock the achievements for insane, the ones for hardcore should unlock as well.
Step 3: Multiplayer
This method will quickly become your best friend if you can get enough people to do this with. First of all, you are going to need 6 people (including yourself, 5). Next, tell them you are going to host a game (this will net you "A Series of Tubes" much quicker if you host) on any map you are missing for “Around the World” (excludes DLC maps) and set it to 18 rounds, and 1 minute round duration. What you should do next is go to a designated meeting spot in the map, preferably the middle. The next part is probably the most confusing concept to understand at first, but you will pick it up after a while. Now that everybody (COG and Locust) are facing each other, have a COG member go out and kill 2 Locust members, and let the third one live. The Locust that is alive will go and kill the two COGs that did nothing that round, and then the COG member will kill the Locust member. Next two rounds, repeat the same thing. To keep on track, the score should be 3-0 COG, with a COG member with 9 kills and a Locust with 6 kills. Round 4, you will repeat the same step again, but have the Locust with 6 kills get all 3 kills this time. At the end of Round 6, the score should be 3-3, and the COG member should have 15 kills, same with the Locust. Next 3 rounds, repeat again but have COG get the kills again. End of 9 rounds, 6-3 COG, COG guy has 26 kills and Locust guy has 21. Switch up again for the next 3 rounds, and now you have a tie game at 6-6 with two guys with 30 kills. Repeat this process except with a fresh batch of players getting kills this time. It should now be 12-12 once this is complete. Lastly, the most important 2 people are alive. Let them get there first set of 15 kills, and it’s now a 15-15 game. Next up, Have the COG win the next two rounds, (with each COG and Locust still getting there 3, and 2 kills respectively)
For this next round, have the COG only kill 2 Locusts this time, allowing the Locust killer to also get his kills, and tie up the round. Once it is tied, the COG should have 23 kills, and the Locust should have 21. The game should also be 17-15 for the COG. Once the Locust now gets the next 2 rounds, (and another 6 kills) The game should look like this: Game tied at 17-17; Every COG has 30 kills, except for one (27), same with the Locust. Now, the Locust and the COG player will both get two kills, and instead of tying it up, will kill each other. Have one player pick up the frags, or another thing that can kill yourself and the enemy (Torque bow, Boomshot, Hammer of Dawn) and kill each other. There you go. 30 kills, 30 deaths, 30 more kills for whatever weapon achievement you were going for.
'Hidden Fronts' Multiplayer Map Pack 2 + Tilte Update DLC
The fastest way to do these is to set up a local play game and sign in a guest on another controller. Now just boost them until they are complete.
[XBA would like to thank INFERNOII for this Roadmap]