For this solution, you have to finish all four LASO lists from the four main games: HALO 1, HALO 2, HALO 3 and HALO 4. You must finish the list available in the game's lists for all of them: setting up a game with the skulls won't count!
LASO is by far the hardest mode in the game. It is in legendary difficulty, plus all skulls on. Each skull adds a modifier that makes the game more difficult, such as less ammo in weapons, enemies with more health and so on. Each game has its on set of skulls, even though some of them appear in more than one.
There are good guides for each of the LASOS in their respective achievements, and it would be too extensive and repetitive to write them all here again. These are the LASO specific achievements for each game. Getting this achivement is, in fact, getting this four achievements:
Here are some general tips:
- If you are playing solo, you can use save and quit to go around the iron skull in all games. When you die, just save and quit before the game restarts, and when you resume, you will be in your last checkpoint.
- Playing with someone can make it a lot easier, as long as both know what they are doing. Remember, if your partner dies, you both go back to the last checkpoint.
- Use some kind of tape or post-it to mark the aim in the games that have a blind skull. Just load a normal game first, mark the aim with a piece of tape or post-it, and then play the LASO.
- Use the skips and despawns shown in videos. They are what makes some of the levels possible and much easier.
Halo 2 is considered the hardest of all, and Halo 4 the easiest. Halo 3 and Halo 1 are close in terms of difficulty and each player has its own opinion about which is harder. But don’t get the wrong idea: all of the LASO are really difficult, time-consuming and they will be quite a challenge. Good luck and don’t give up if you are going for this.