- Estimated Achievement Difficulty: 8/10
- Offline: 44 
- Online: 35 
- Approximate time to 1750: 50+ hours
- Minimum number of playthroughs needed: 1
- Number of missable achievements: 10
- Do cheat codes disable achievements: N/A
- Does difficulty affect achievements: Yes
- Glitchy achievements: None
- Unobtainable achievements: None
- Extra equipment needed: None
Welcome to Halo 3. There are a total of 79 achievements worth a total of 1750. It is possible to get all the campaign achievements in only one playthrough, but you'd have to play on Legendary, with the meta game scoring on, and looking for skulls/terminals. I recommend two or three playthroughs.
There are ten missable achievements: you have to find the hidden skull on each of the nine levels, which is one achievement per level, and you have to access all seven of the hidden terminals, which makes ten. You will unlock more than half of the achievements, 1100 offline, in either Campaign or Forge. The other 650 are online multiplayer achievements which can all be boosted.
Step #1 [Optional]: Campaign on Normal
Please skip this step if you are familiar with FPS games.
Reload (pick up gun/switch gun/get in vehicle/etc.)
If you are new to Halo or even FPS games as a whole, I would recommended playing on Normal for the first time. It will still get you a minimum of 405. You may or may not get the following achievements: "Too Close to the Sun", "We're in for some Chop", "Fear the Pink Mist", "Headshot Honcho", and "Used Car Salesman".
Step #2: Campaign on Legendary [Skulls/Terminals included]
I strongly suggest you play this in co-op instead of solo. It is much easier. It can be done online or local splitscreen. You will also be finding the skulls and terminals as well in this playthrough.
If you have two controllers but no one else to play with, you can make a co-op game with a guest and just leave the controller alone; every time you die you will spawn by him, and when you go too far ahead the game will load him next to you.
Refer to the guide for the locations of all the skulls and terminals.
Once you have beat Legendary, you will have a minimum of 785.
Step #3: Campaign Meta-Game
At this part you should have just about all the campaign achievements. To make it short, you are required to score a certain amount of points on each campaign mission. It's not the same on each mission.
I recommend playing on Normal with the skulls: Grunt Birthday Party, Tough Luck, and Catch. But of course it's all personal preference, so choose whichever skulls you are comfortable playing with.
Here is an outline of the meta game scoring.
Step #4: Online Multilplayer Achievements
Now, you should have all of the campaign achievements with the exception of Too Close to the Sun, We're in for some Chop, Fear the Pink Mist, Headshot Honcho, and Used Car Salesman. You should have a minimum of 875. There are 35 online multiplayer achievements worth 650. Some of them can be tough, but they can all be boosted. All ranked achievements, except MVP, must be done in the Lone Wolves playlist. All Legendary and Mythic pack achievements can be done in ranked or social matches in any playlist.
Step #5: Forge
This step should not take long. Simply go into Forge and play on each of the six Mythic maps: Longshore, Heretic, Assembly, Sandbox, Orbital, and Citadel. You will get the achievements for finding the skull in each map, each worth 25 and the achievement for finding all six skulls.
Refer to the guide for the locations of the Mythic Skulls.
The last Forge achievement requires you to kill someone while in monitor mode on a Mythic map. On any of the six Mythic maps spawn a tank and kill someone while holding it by slamming it into them.
Step #6: Mop Up
At this part, so you should have the majority of the achievements in Halo 3, if not all of them. The only achievements I can think of that you will still need to get are Too Close to the Sun, We're in for some Chop, Fear the Pink Mist, Headshot Honcho, Used Car Salesman, and Vidmaster Challenge: Annual.
Extra Info for the Extra 750G:
- The Mythic Maps are: Orbital, Longshore, Heretic, Citadel, Sandbox and Assembly.
- The Legendary Maps are: Ghost Town, Avalanche and Blackout.
- None of these achievements can be gained on Heroic Maps, release maps or Cold Storage.
- Here is a list of the possible gametypes in each playlist: https://www.bungie.net/online/playlists.aspx.
Halo 3 is overall a really enjoyable game to 1750. The only ones you might struggle with are the multiplayer achievements, but you can always boost them if need be.