Vigor
30 Achievements
Last Survivor
Leave the level as the last
17.2%
How to unlock the Last Survivor achievement in Vigor - Definitive Guide
Note, keep trying this method because it definitely works better than going around and killing everybody yourself sometime. And I don't know how, there's people that might still be left alive somewhere but it should work 90% of the times.
Note 2 : if the furthest exit on the map is close to a signal detector and it flashes green, you can activate it when the storm is getting closer to have an idea of how many people are left and how many are inside the storm already.
At any point during an encounter, you can head to an exit area to leave. The game does not have an on-screen number of how many people are on the map, but you can use the Signal Detector (marked on your map in every encounter) to see enemies on your map for a short time. The Signal Detector has a cool down time, but can be used infinitely. Wait until there are no more enemies on the map.
You could also just camp by an exit area that is located opposite of the radiation. The radiation is shown on your map with red arrows and does a small amount of damage per second as it covers the map. Radiation begins to cover the map about 9-10 minutes into the encounter. Wait for the entire map to be covered by the radiation and then escape. If this is your first time being hurt by the radiation you will unlock "Radioactive." By this point, everyone would have normally left the map and you should be the last Outlander to leave.