If you're experiencing motion sickness from the shaky camera or it's just getting on your nerves 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 X button to enter controls and select the Steadycam option. Use the Left Stick to enable this option and then press the X button again to confirm.
When you throw a fire extinguisher it 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 R1 and the auto-aim will take care of it. This holds true for other thrown objects, such as fuel cans.
At the tunnel stage in the chapter 'Laying Low' where the officers appear at the other end you can get rid of them by throwing the gas can toward them and and shooting 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.