Getting this achievement might seem confusing at first glance but it's actually quite simple, You just need to kill everyone when the map size is large meaning when The infection rate is between orange and red without using The Virus(num 5) , I recommend using special zombies such as Jason and Freddy Krueger or/and normal spawn zombies. Also using consumables is a must especially the canister because that's the only way you can besides normal zombie spawn.

I Hope that I helped You with this little guide and I apologise if I made any mistakes please share Your views if You'd like, well then Goodbye.