This is the final chapter of the game. You must fight off Server Attack Drones until the lift reaches the top, at which point you will engage in the final confrontation of the game.
Adrean will start by hovering in the air and tossing 'bug bombs' at you. They appear to be capsules containing little Seed Spawns. Continue firing at her with any of your weapons until she flies down and lands on the ground. Her wings covering her will render her immune to attack. She will then spit this acidy substance on you, you can dodge it easily. You must dodge 3 jets of this until she relinquishes her defensive position, allowing you to damage her again. She will leap into the air and repeat her 'bug bombing' procedure. You can only damage her when she is in her bombing run. Repeat this damaging procedure until she falls, Nathan will then come to the rescue and the final cutscene will commense, after this, the game will end, congratz, you've beaten Total Assault!