If the shaky camera is getting on your nerves (or you're experiencing motion sickness), you can enable a Steadycam mode in the 'Options' menu. This is done by pausing the game, entering 'Options' and highlighting the Controls section. Then, press the A button to enter controls and select the Steadycam option. Use the Left Stick to enable this option and then press the A button again to confirm.
A thrown fire extinguisher will bounce and travel quite far, so give it time to reach your target before shooting to detonate it. There's no need to take aim when shooting to detonate an explosive; just hit RT and the auto-aim will take care of it. This holds true for other thrown objects, such as fuel cans.
When you reach the tunnel where the officers appear at the other end in the chapter 'Laying Low' throw the gas can toward them and and shoot it once it reaches them. Then get behind cover and use your shotgun and rifle to kill them and clear the way forward to the next area.