- Estimated achievement difficulty: 9.5/10
- Offline: 17 
- Online: 9 
- Approximate amount of time to 1000g: 80-100 hours boosting, 500-1000 'legit'
- Minimum number of playthroughs needed: 1 plus a few mission selects
- Number of missable achievements: Grizzled Veteran is 'missable' if you don't do the campaign on Veteran in one run-through [you can save the game, shut off your 360, or whatever, you just can't start a new campaign or chapter select midway through].
- Glitched achievements: None!
- Do cheat codes disable achievements: No Cheats
You will have to play through the game on the hardest difficulty, which is infinitely easier said than done. You can attempt to play through some missions in certain ways for 'side' achievements, but it's a lot less taxing to do it later on the lowest difficulty.
As far as multiplayer, the game requires you to get 40,000 online points, which is amazingly time consuming, even with boosting. There are a few 'side' achievements, but they're nothing compared to General.
Call of Duty 3's achievement list won "Hardest Shooter 1000" and "Most Time-Consuming Single Achievement" in x360a's 06/07 Achievement awards, so be warned.
First off, start up the game on Veteran. This is the hardest difficulty setting. It's not hard in the traditional sense, but it's brutal because of how buggy the game is. Enemies will shoot through walls, only at you, and with perfect aim. Movement is awkward when crouching/lying down, but rest assured, you will never get stuck in a wall or behind a door or anything that will force you to reset the mission. Nevertheless, the campaign is doable after 15-25 hours of frustration, depending on your skill. Make sure you start the game by selecting "New Game", and to always continue the game by selecting "Continue". You must beat the game on Veteran in one "run-through", otherwise "Grizzled Veteran" will not unlock. Be sure to read the tips in the guide. Beating the game on Veteran will get you a minimum of 7 achievements for 300.
Get the "side" achievements for singleplayer. You may have already unlocked a few during the Veteran run, but it's a lot easier to go for them on "Easy". Use Chapter Select to play the chapters named in the achievement guide. You will get the remaining 10 singleplayer achievements for 275.
Welcome to General, one of the most brutal achievements ever. Every match in Ranked multiplayer is 20 minutes long, though it may end earlier if one team gets a certain amount of kills or captures the flag a certain amount of times, depending on the gametype. A player would be lucky to get 40 points in a match. Adding in lobby time, you can get 2 matches in per hour. That's 80 points an hour, meaning General will take you 500 hours. The good news is, that there are zero bugs on the achievement. All points (unless you leave or get kicked because of teamkilling) will count on the leaderboards, and General will unlock at 40,000 points exactly.
Once you hit 40,000 points, you will get at least 5 achievements worth 320 points.
However, there is a way to boost:
Step 1.1 (optional):
Use the forum's Achievement Trading Thread to get a group of people who want to boost. 8 or more (you need 8 people to play a ranked match) is best, but about half of all 'random' people on COD3 are willing to boost. Be sure to make a silver account named "COD3 boosting" or similar and use the FoF feature to have a convenient list of names.
You can always go into games by yourself and try to get everyone to boost, but it's a major pain to get all 7+ other people to comply.
Step 1.2 (optional):
If you have "boosting partners", start up a private player match. (Battle is best, because everyone will be able to talk to each other regardless of team) Then, assign one person (PlayerA) to scout and one person (PlayerB) to be on point. Have PlayerA and PlayerB private chat each other, and PlayerA leave the game and begin to search for ranked capture the flag matches. PlayerA needs to wait until he can get into a lobby by himself or close to it. When that happens (and it can take a long time, depending on traffic), PlayerA tells PlayerB to leave and search. All of the other boosters need to know to search once PlayerB leaves.
As the lobby begins to fill, ready up and make sure everyone knows that you're boosting. Ideally, you can get in a room composed of nothing but boosters, but more often than not, there will be a person or two who can't speak English (or whatever language you're using), doesn't have a mic, or will intentionally screw you up. Usually, the first two types of people will figure out what's going on after a while. Have patience.
Once the game starts, all boosters should automatically spawn as Medics and switch to pistols so it can be easily seen who's complying and who isn't. Err on the side of caution, because one accidental killing can mess everything up. Have one person on each team go to the other team's base (throwers), and everyone else needs to go to their base (catchers).
The throwers need to pick up the enemy flag and drop it (with the same button that fires your guns) onto the catchers. The flag needs to be about one body length away from the base or the flag cannot be caught. They need to do this repeatedly for the duration of the match. The idea here is that each thrower is picking up the flag for one point, and each catcher is returning the flag for one point. There will be multiple catchers, but as long as the catchers are touching each other, they should all score a point.
The main factor in the amount of points you will score (besides people not complying) is the skill of the thrower. The best throwing technique is to stand with your back to the flag, pick it up, then drop it. Ideally, no movement is required, but most maps will 'push' you with every pickup/throw. A good thrower can get 500 pickups in a match, meaning everyone can end the match with 500 points. However, the game can get overloaded from all the flag activity and begin to lag. The more lag, the more you will score from each individual pickup/catch. Getting 1000 points in a match is not unheard of.
Once the match is over, one team will win based on points. (it has no bearing on the achievement though) Once the 30-second timer ends, everyone should immediately search for a Ranked Capture The Flag match and start again. The best way to boost is in several long stretches of 5-10 hours each, as it's simply more convenient to keep rejoining rooms than to set up player matches.
In addition to General, there are several "side" achievements. There are 4 achievements worth 105. "Victory Medal" will most likely be achieved during General boosting, and you can get "Supply Officer" during a boosting match if you spawn as a Support class and keep dropping ammo on catchers. "Doc" can be easily attained by playing as a Medic during a match and just running around reviving. "A War Hero" is the only tricky one, as it must be done in a War game, which you can't General boost in. Really, all you have to do is win a match of War and make sure you're there when the last flag gets capped.
In all, these side achievements are nothing compared to General and can be easily acquired in one day. Plus, you can play the game 'for real', and COD3's quite fun.
[XBA would like to thank Marx0r for this Roadmap]