This chapter starts off quite atypically. The large spider-like creature must be fought off; pump a few magnum rounds in its head for starters. There's a lot of ammunition on the bus which you should all collect while you have the chance.
The creature has several attacks. Its main attack is to simply slash down at you, which can be avoided by sufficiently moving around. It can also give birth to larvae that move slowly but surely towards Castellanos. When the creature comes at you a second time, it will also use a slash attack from the side. Ducking will do the trick this time.
The team runs into several regular enemies too. The idea is quite simple; kill them all. There are flammable barrels all over the place, so use these to your advantage. Give priority to gun-wielding enemies.
After the scene the game resumes its more traditional gameplay. You have to make your way to the ambulance. There are plenty of flammable barrels to mess up enemies, but be careful as many have weapons. When you approach the far side of the area, a sniper pops up in the distance. When he's taken care of, continue to the ambulance and collect the [SYRINGE] and [HEMOSTATIC].
Head back to the bus, which is easier said than done. A jeep with a gunman behind a turret (REALLY!?) shows up. Hide behind cars and make your way to the jeep, then shoot the gunman (when you're standing behind him). Quickly seize the turret and waste several waves of enemies. Keep in mind that the crosshair's middle circle is the actual place where bullets are fired at. In any case, once they're all dead - bad choice of words - head back to the bus. A short and unnecessary driving scene ensues, after which the chapter ends.
Map fragment is in the ambulance
Very thorough guide, nice 1