Conspiracies here bringing you another guide:
Have every territory saved at the same time during the Undead Nightmare. (21)
Every town and some of the gang hideouts are now overrun by zombies, and it's your goal to have them all "cleansed" at the same time. If you get near a territory it will appear on your map with the status "Unknown". If you approach it the status will be "Overrun".

If you capture a territory for the first time all survivors (amount depends on size of the territory) will be alive. Though after a certain amount of time the territory will be overrun again and the status will change to "Under Attack". If you won't help the territory it will become "Overrun" again and survivors will start dying until none of them are left. Fort Mercer, Casa Madrugada and El Presidio are always "safe".

A territory will take about 30-72 hours (in-game) to become attacked again, so you'll have to move quick. The best way is travelling by horse as fast-travelling takes a longer in-game time. There are 13 territories in America and 9 in Mexico.

Even if a town is overrun and all survivors are dead, you can still unlock the achievement and save the towns.