The Flood (level)/Walkthrough
From Halopedia, the Halo wiki
You begin the level at your base, with various supply pads already running in it. In this case you shouldn't have to worry about building new ones unless you want to, as you do not really need them. This level can be very easy and short or very long and frustrating, so right now we are going to cover "easy and short" method. This method involves using Hornets to eliminate any Flood that try to get in your way, allowing for a clear path towards the Proto-Gravemind.
As soon as you get the resources and the technology levels are upgraded build an air pad. You may want to build all your turrets as well, and upgrade them to a Flame Mortar, as the Flood will be constantly sending their forces at you, and the Flame Mortar should make short work of them. Once you get the resources, create all the Hornets you can make, around 10-15 would work fine, but if you want it done quicker, you should build more. Traverse the terrain, taking out any Flood you see. The main goal is to kill the Proto-Gravemind (The structure that is labeled as the "Flood boss".) An easy way to do this is to kill the multiple Flood Colonies around the map. With a large squad of Hornets, you can get around the map a lot quicker and get all of the Colonies while they are down. There are lots of other structures around the colonies you can ignore them.
You could either kill every Flood Colony, or you could kill a few and bombard the Proto-Gravemind with everything you have. You can kill the Proto-Gravemind by simply attacking it, but every time you kill a colony it's health slowly decreases. If you somehow kill all of the colonies before they regenerate, the Proto-Gravemind will automatically die.
If you want, the quicker way would be to kill a few of the colonies and then go for the Proto-Gravemind. The Proto-Gravemind will fall quickly, though this is depending on how many Hornets you have and how many colonies you have already killed, but many people have only killed two and were still able to destroy the Proto-Gravemind in about 1 minute with 10 Hornets. It is all up to you.
Keep in mind that the Flood Colonies will eventually regenerate over time, and it will be a challenge keeping them all dead at the same time. It is completely all up to you, so do whatever you think is right. As long as you have a great supply of Hornets this level may be one of the easiest in the game.
After the Proto-Gravemind has been destroyed, the remaining UNSC survivors will get out of the area and prepare for an evacuation, thus finishing the level.