Halo Wars: Definitive Edition

There are six airlocks in all: two for Hornets/Gremlins/Wolverines, and four for Marines/Flamethrowers/Cyclopses. Any one of these gets KO'd, you need to resign and start over.

This achievement can be done on any difficulty, whichever you prefer. I did this on Normal after 3 tries; first try I let an airlock get destroyed without realizing it until it was too late, and the 2nd try I let the Flood generate too many enemies at the rear to stop them in time.

You start off with 3 Spartans, Sgt. Forge, a Cyclops, some marines and flamethrowers, and a Hornet; at least 10 units total (Maximum of 30 units). The mission takes place on the outside decks of the Spirit of Fire, and there are LOADS of Flood trying to do some damage. You can build Turrets at no fewer than 10-12 locations throughout the map of the mission. Your only Command Powers are Carpet Bomb and Regeneration, with no option to upgrade either of them.

The good news is you have a slow but steady flow of resources coming in throughout the mission, so no need to worry about building supply pads (you couldn't make them anyway in this mission). There are a few supply crates to grab at the front-right side of the ship, if necessary, to quickly boost your bank for whatever you need (more units, Turrets, upgrades, etc). You also can freely upgrade your units sans the Cyclopses without needing any reactors (again, you can't make any to begin with).

The bad news is that the Cyclopses CANNOT repair any damages done at all. You have to use Regeneration to heal any damage done to your units, Turrets, and airlocks. Don't waste your resources making an army of Cyclopses, as all they'll be able to do is fight enemies.

The goal of this achievement (and the mission) is to take out ALL of the Flood - from the moveable ones to the stationary ones marked by all the RED markers on your minimap. The stationary ones you really need to worry about are the ones that generate mobile Flood (Infection pods, flyers, infected soldiers, etc.), small AND large, and the ones that spew projectiles at your units.

Crank out 1-2 more Hornets when the mission starts and split up your units to tackle the nearest Flood units. Set up at least one Turret (preferably one between the two Airlocks at the front of the ship) and assign most of your units to the back of the ship, where the most Flood likely spawn from. Carefully assign different airlocks to upgrade your units - I recommend only doing Wingmen, Oxide Tank, New Blood, and Rocket Launchers only until you have built up enough resources. Make at least one or two Turrets to cover the back airlocks while you focus on weakening the Flood at the rear of the ship.

Eventually you'll get warnings that 1) Sentinels are joining the fight to "help" cleanse the ship, and 2) a cleansing wave system is activating to work on the Flood on the ship's outside. The latter will occur every 3 minutes once it completes its run on the ship, and it will damage your turrets and your units. You CAN put most of your units inside airlocks to keep them safe IF you put them in their correct airlocks (Human units in Human airlocks, Wolverines/Gremlins in vehicle airlocks) in time. Sadly, all flyer units can't go into airlocks, but they will survive the beam with only some damage (Serina tells you to fly in the opposite direction of the beam). However, the beam WILL take out Flood Stalks and Flood pods as well as all Flood enemies automatically. Flood generators MUST be at near-zero or absolute zero energy to be eliminated, otherwise they will regenerate their health and revive themselves to further plague you.

Sentinels will attack the airlocks and your units, so you will have to take them out as well in order to protect your assets.

Once the rear of the ship is cleansed of all Flood, focus on the front while also keeping an eye on your airlocks. Flood will rain down on the ship, so again spread out your units to cover the airlocks. Have at least 6 Hornets, and at least 2 Wolverines, and if you have the resources, finish upgrading marines, flamethrowers, and Hornets fully. Use regeneration on Airlocks and units when necessary, Turrets secondary. If you can, try to group your near-dead units with an airlock and hit them all with Regeneration.

Hornets are the real key here, because Flood generators will be on both sides of the ship, and only Hornets, plus two Vultures sent out to join you if you hold out long enough, can reach these areas. Assign at least 2-3 Hornets and 1 Vulture to spots on the ship's sides to take out and watch these Flood generators until the cleansing rings activate so they can be eliminated for good.

Once the hull is totally clear (Absolutely NO red dots at all on your mini-map), Forge will give the all clear signal and the mission will end. If you have all six Airlocks still intact (you can press to summon units if you move the cursor over them), then the achievement will pop as soon as the mission's scoreboard comes up.

Good hunting!

Best way to do this is to make a bunch of hornets and turrets and try to clear the ship of flood in as little waves as possible. You need to try and kill anything attacking the airlocks, and heal the airlocks with your repair power if they get low. This can be done on any difficulty, but obviously it will be easier on easier difficulties.