After the cinematic ends, you'll find yourself being driven around in a school bus. The top is ripped off by that big spider creature, and he's shaking the bus. You can't go anywhere while he clamps on, so shoot him until he lets go. Joseph will help you here, so let him do most of the shooting, as he (and your other partner, when she shoots) has infinite ammo, while you obviously don't. Don't expend your powerful weapons at this guy; just a few pistol shots shout be fine, as you can't kill him. Be wary not to be stabbed by his legs, and shoot the little gross maggots he spawns from his underbelly when they appear so they don't damage you, as well. When they are shot, they give pistol ammo, but it they are meleed, they give nothing. When he finally lets go, ride the bus down the streets until your driver parks in a building.
Here, you will be trapped in while all sorts of Haunted come in a throw projectiles at you. Mostly they are Molotov cocktails, though there can also be some hatchet throwers, as well. While you can shoot, your partners will also be shooting, so remember that they will always have ammo, so let them do most of the combat. Eventually, some zombies come in and stand over the roof of the driver's seat, and they carry Sniper Rifles. Your partners don't seem to be able to hit these guys (due to their position), so feel free to shoot them, yourself. At some point, an enemy in the front of the bus will be shot, and you'll see a short cinematic of him dropping his dynamite under a gas truck, thus exploding it and clearing the way out for you.
When you're on the streets again, that big monster will come up at some point, and you'll have to shoot him off again. use the same tactics as you did at the start of the chapter. When he's gone (finally), watch the cutscene. After, you'll have to get a hemostat from the ambulance for your best friend, but unfortunately the area where it is parked is patrolled by lots and lots of zombies. There are lots of barrels here to be used (i.e., shot at when an enemy is close to them), and you can also attempt to sneak kill a few, but since most have guns and dynamite and have a high chance of spotting you, your best bet (if you don't want things do devolve into a massive gun fight) is to use flash bolts from the crossbow, then kill with a sneak Knife stab. There are also a few bombs scattered around. You can shoot them when a zombie is close to them, but you can also disarm them. You'll likely need the parts for the next part of the chapter, anyway. Be careful, as when it seems that all zombies are gone, there actually are two more. One hides behind a truck, and happens to be standing right next to a barrel, perfect for a barbecued zombie, and the other stands on a concrete mound near the ambulance. This one has a sniper rifle, so shoot him from a distance with your own, then go to the back of the ambulance and get the glowing blue first aid pack. There is also a map fragment in the back, as well, the only collectible for this level.
With the pack in hand, run back and you'll see a military vehicle speed up and the zombie at the back will begin to shoot a very high caliber mounted gun at you. Don't hide behind the cars! They can blow up if he shoots at them, dealing you heavy (if not lethal) damage, so hide behind the concrete piles. Wait until he stops shooting, then run up to the back of the truck. Here, you'll find about 8 or so zombies waiting to kill you. You can blind them all with a flash bolts and sneak kill them all, or electrocute or freeze them at kill them in other manners. At any rate, get rid of them so you can kill the guy still on the back of the truck. Hop on, and get ready to have some fun, because for one of the few times in the game, you don't have to worry about ammo! The gun has an infinite magazine, so let 'er rip on the waves and waves of zombies running up to you. At some point, it'll seem like they are all gone, but when you get off, more waves will spawn, so get back up on the gun and continue shooting. Be aware of the zombies that like to crawl up the side of the truck, so look 360 degrees to make sure there aren't any enemies that want to expel you from your position. There may also be a few zombies that will run up to the bus and start shaking it. Shoot these, too. When there are no more enemies to be shot at, leave the gun and run back to the bus, grabbing up whatever the zombies may have left behind.
After the small cinematic where you save your friend, you can actually drive the bus down the road. It is of note that there are multiple zombies shuffling down the road, and in order to get the achievement "Not a Scratch on Her!" you have to not hit a single zombie. Cars seem to be okay, but even brushing a zombie disqualifies you from the achievement. As you drive down the road, Ruvik will appear and levitate the bus, throwing you through a building and ending the chapter.