Alright you need to infect the chainsaw guy first. Second you need to clear out the first building (caution: Don't go fast or be careless that's how the humans kill your zombies) Once you get to the second building don't get a huge group of zombies instead let one through and add a tank to kill the first ceo third add a crawler and quickly make a overlord. You may need to let your tank die and let the crawler fall in the vent, kill the worker, kill the cops, add a tank to finish off the second ceo. (The third ceo is a little challenging be smart about this) Once you get to the closer kill count you need to kill 2 more humans in order to complete the objective. Make a crawler get to the top and QUICKLY explode near the humans it's a bit challenging because they sometimes runaway. After you manage to kill the remaining humans you are ready to go to the next objective. Hope this helps you