From Halopedia, the Halo wiki
All Alone is an achievement for Halo: Reach multiplayer matchmaking added to the Defiant Map Pack with the release of Halo: Combat Evolved Anniversary. The achievement was earned by being the last surviving uninfected player in a matchmaking Infection game. It could only be unlocked on a Defiant map.
The achievement is worth 10 Gamerscore.
Arguably the most difficult part about earning this achievement was finding an Infection match on a Defiant map. The Living Dead playlist contained 21 different maps, only two of which were Defiant maps: Condemned and its variant, Uplink. These maps would only appear if all players in the party had downloaded the Defiant Map pack, and even then there was only a 28.6% chance that one of the maps would appear in the voting queue. Even if one of the maps did appear, other players could vote for another option.
In the lucky event that the player managed to find themselves in an Infection game on Condemned, arguably the best strategy was simply to hide and wait for all the other Spartans to be infected. This would lower the player's overall score, but drastically increase their chances of unlocking the achievement. Condemned features numerous corners and out-of-the-way areas. One excellent hiding spot was on top of the Sabre in the Repair Bay. This provided a dark hiding spot and allowed the player to flee in any direction if they were discovered.
Upon finding their hiding spot the player was wise to remain completely still as so not to show up on other players' motion trackers, not shoot at the Infected, move around or otherwise draw attention to themselves unless it was absolutely necessary to prevent themselves from being infected. It was advisable to avoid not only the Infected but also other Spartans, as their presence could draw attention, and once they became infected they would likely remember the player's location and seek them out. With any luck the other Spartans would adopt more aggressive tactics and soon find themselves infected, leaving the player as Last Man Standing and unlocking the achievement.
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