Survive the city lockdown Niemand starts once you eliminate the last of the 5 underbosses (as in, Berg, Roxy, Keita, Khan and Sorensen) I had bad luck with Roxy as my last underboss as her tower was a bit difficult to defend. I'd consider either a different boss as your final one or immediately leave for the ground once you terminate the final underboss. Make sure you are somewhere you can defend with easy reloads as its a pretty heated battle. There are reports that the 'final' lockdown can be triggered by one of Niemand's direct underlings as well - so YMMV with which lockdown pops the achievement.
If you didn't survive the 'story mode' lockdown, you can trigger the final (achievement related) lockdown by picking a battle with one of the faction groups and pulling that battle to another group of faction enemies (not the easiest to do in practice as this means you need 2 different kinds of enemies on you and they don't typically spawn near each other. Additionally, you can trigger this lockdown again by listening for word of a supply convoy in your area. Be armed appropriately and take it on! Do not relent. Keep destroying unit after unit the city is put in lockdown. Survive this lockdown and you'll unlock the achievement. I was near the Chem plant but you can presumably find convoys anywhere on the map. It does not matter if you've removed Niemand or not. The achievement related lockdown will happen regardless.Spazpol
suggested stealing a tank as the added firepower makes easy work of the forces sent at you.keung
noted that if you are playing in co-op and your partner dies, this will void the attempt. Neither you nor your partner can die.