You can mass produce Banshees thanks to the Pain train skull (it takes like 2 seconds to build a Banshee without the logistics upgrade). The army you want to build consists of 5 engineers and the rest will be all Banshees. By this time the flood will probably not even attack your base anymore (after a few waves they just don't care anymore and you have all the time to build your army).

Get your bases to level 2 a quickly as possible and build an apex and a war council.

What you want to do is build 3 harvesters and 2 power extractors and upgrade these. Don't worry about this because the army you already have is more than capable of dealing with all the flood attacking your base. Once the mission starts, you will be attacked by the flood. To make this achievement a lot more easy, you can also use skulls that make the game easier like Pain train (faster unit building times), Bountiful Harvest (more resources) and Emperor (faster leader powers). I chose the Nightmare skull since it doesn't really do much except unable you to save your game. The way this achievement works is that you can turn on all the skulls you want if at least 1 of them is a skull that increases your score.

The par time of this mission is 30 minutes, which is easily doable.

The mission I did this on was Manifestation (the last mission). Before I start with the solution, I want to note that this will include spoilers from the last mission of the Awakening the Nightmare DLC.

Banished pc game sequel