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 is earned by being the last surviving uninfected player in a matchmaking Infection game. It can only be unlocked on a Defiant map.
Arguably the most difficult part about earning this achievement is finding an Infection match on a Defiant map. The Living Dead playlist contains 21 different maps, only two of which are Defiant maps: Condemned and its variant Uplink. These maps will only appear if all players in the party have downloaded the Defiant Map pack, and even then there is only a 28.6% chance that one of the maps will appear in the voting queue. Even if one of the maps does appear, other players may vote for another option.
In the lucky event that the player manages to find themselves in an Infection game on Condemned, arguably the best strategy is simply to hide and wait for all the other Spartans to be infected. This will 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 is on top of the Sabre in the Repair Bay. This provides a dark hiding spot and allows the player to flee in any direction should they be discovered.
Upon finding their hiding spot the player should remain completely still as so not to show up on other players' motion trackers. Do not shoot at the Infected, move around or otherwise draw attention to yourself unless it is absolutely necessary to prevent yourself from being infected. Try to avoid not only the Infected but also other Spartans, as their presence may draw attention to you, and should they become infected they will likely remember your location and seek you out. With any luck the other Spartans will adopt more aggressive tactics and soon find themselves infected, leaving the player as Last Man Standing and unlocking the achievement.