For this trophy you only need to extinguish 10 fires but for the 100 percent trophy you will need to complete all 12 levels of the firefighter activity. You have to worry about tipping the truck, the timer and cops; not to mention the fact that the morons on fire run away from you in all directions.
However there is one sure fire way to make this easy as possible. What you want to do is steal the truck and then drove to the airport before starting the mission with R3. With a little luck the game gets confused and all the cars for all 12 levels spawn right there on the open tarmacs. Furthermore some of the morons on fire can run to the nearby water and extinguish themselves. All you have to do is keep firing your water forward and follow those that don't from behind. Easy.