Set up your squad so you have 3 Flamers on hand, with each character bringing a Chainsaw and Electric Armor. At the start, split up so one marine wipes out the upper-left eggs and the nearby biomass, another wipes out the biomass and eggs in the middle, and the last one (preferably Wildcat or Faith for their Speed Bonus) rushes through the ducts on the right side. Activate Electric Charged Armor to destroy attached Parasites or prevent them from latching on to you. The fourth marine should set up at the elevator and press the button as soon as all the objectives are completed.
Set Biomass and Eggs on fire with the Flamer, but don't wait around for them to burn down, run to the next cluster. The only exception is biomass that blocks your way: keep up a steady spray of napalm so it goes down faster. Meleeing burning eggs destroys them faster as well.
Running through the flames and then extinguishing yourself is faster than extinguishing the fire itself.
Enter the ventilation ducts via their south entrance, shoot the egg with your Rifle/Shotgun and then turn north. As you approach the biomass, activate your Smart Bomb to destroy the eggs in the southern half of the ducts. Sustain fire on the biomass until it is destroyed. In case Faith's Smart Bomb fails, continue north and turn west to destroy any remaining eggs. Meet up with your team at the elevator.
Before entering the junction where you turn east, destroy all the Drones and Parasites in your path with grenades before they can obstruct your team. Run to the north entrance of the ventilation ducts, destroy the eggs there with grenades, and sustain fire on the biomass. Just after you enter the vents, activate your Smart Bomb and run east. If you're fortunate, then the missiles will destroy all of the eggs in the northern half of the ducts. If not, then destroy the remaining eggs on the eastern side. Meet up with your team at the elevator.
Helper 1 (top-left)
Ignite the biomass in the mall, backtrack to go upstairs, and run north while igniting any other eggs along the way. A Smart Bomb can be activated to kill the Boomer in your path. Once the northernmost eggs are in your range, shoot them through the locked door with your Marksman Rifle. With enough practice, you can shoot all three eggs without having them in your view. Meet up with your team at the elevator.
Helper 2 (bottom-left)
Go to the biomass in the central area and drop a Tesla Coil for cover. Sustain fire on the biomass until the posterior section has been directly exposed to your Flamer for approximately three seconds. Run to the elevator switch and hit it as soon as possible. Drop your remaining Tesla Coils near the elevator entrance, and survive as your team arrives.