- Estimated achievement difficulty: 7/10
- Offline: 20 
- Online: 21 
- Approximate amount of time to 1000 : 35–50 hrs
- Minimum number of playthroughs needed: 1
- Number of missable achievements: None!
- Glitched achievements: None!
- Do cheat codes disable achievements: No cheats
- Does difficulty affect achievements: No
Estimated achievement difficulty of 7/10 is based on completing the campaign on Legendary. You cannot do it in co-op, but some levels are very hard.
This game has mostly easy achievements, but completing the game on Legendary is tough. You can do the levels in any order, but you must do all levels in Legendary to get "Legend" (50). Doing the game on Normal will net you a maximum of 900. The online achievements are easy and an experienced player could get them all legit. However, you may want to boost for "Decorated Soldier" (10, earn 8 medals in a game), "Killtacular" (20, 4 kills in 4 seconds), "Rainman" (25, play all 47 game variants), and "Violent Cartographer" (25, play on every map).
Step 1: Playthrough Campaign
If you are going for the full 1000, you must start your game on Legendary difficulty. You will unlock all level achievements along the way, and the 3 difficulty achievements at the end (they are stackable). You can follow this guide which helps to avoid battles and plan weapons to bring in boss fights ahead of time. It is very useful and can save you from frustrating fights. Note that the sword teleport trick that is shown in some video walkthroughs doesn’t work anymore with the new update. You can also type “Speedruns” and “Halo 2” in YouTube to find great shortcuts. However, even with the guide, a lot of skill is necessary. Don’t give up on the first level, which is one of the hardest !! While doing the campaign, don’t worry about the other solo achievements, you can always come back later with mission select.
Step 2: Other offline achievements
"King of the Scarab" is done in Metropolis. Just look at the video walkthrough in the guide to reach the scarab gun. Try to complete the level without dying to get "Demon". You will also need to redo parts of "Gravemind" (Go Ape Shiv) and "Delta Halo" (Silent but Deadly). Note that restarting the same checkpoint as stated in the guide doesn’t work, so you can try killing grunts with the tank in Delta Halo. Near the end of the level, at the entrance of the temple, you can shoot them before they see you. You don’t have to kill them from the back, as wrongly stated in the achievement description.
Step 3: Online achievements
To boost for this game, you need a total of 4 people. You should start by trading medals ("Decorated Soldier"), by choosing a Slayer match, on a map with vehicles (e.g., Coagulation) set to 25 kills. You need "Sniper kill" (you can set the starting weapon to sniper), melee from the back (), melee from the front (), clutch plasma-grenade kill (select plasmas with , then press ), road kill (run over someone with a vehicle), hijack ( near a manned vehicle), killing spree (5 kills in a row without dying), and double kill (2 kills in 4 seconds). By doing this, you will also get a lot of other achievements (try to get 10 kills in a row also). The achievement for "Decorated Soldier" unlocks at the end of the game.
You can also get "Killtacular" with only 4 people. To do this, choose a small map like Midship and set the starting weapon to rocket launcher. You actually have 4 seconds between each kills, so you really have 12 seconds to get your 4 kills. Simply kill 1 person, wait a little, kill another, wait a little, etc. By the time you kill the 3rd person, the 1st will have respawned and you can kill him again. Obviously, its easier with 5 people.
Then, you can do "Rainman" (play all 47 game variants), and "Violent Cartographer" (play on every map). You must be 4 people in all games. It takes around 1 ½ hours to do. Make sure you change map and gametype each game, and change the settings so it takes minimum time (e.g., set it to 1 kill, 30 seconds, etc.). You can die, as long as you complete the game (do not leave).
All other achievements can be boosted with 2 people only. Note : If you don’t play with someone who has "Legend" (to unlock the “viral” achievement "Hired Gun"), you can always get it yourself after you complete the game on Legendary. See the guide for more details.
As you can see, Halo 2 has easy multiplayer achievements. However, you will have to work hard for the Legendary campaign. Good luck!
[XBA would like to thank Mtld for this Roadmap]