By the time the game is completed and all other trophies except this is earned, one should be around 1800-2000 mark. More if you had planned to take over every gang territory. For the remaining 2000+ kills, make a save at abandoned air strip in the desert. Take a jet pack and land in the middle of restricted area just west of the air strip. The Rhino tank is parked inside a shelter. Jack the rhino and take it back to the airstrip. you will get 5 stars but doesn't matter with the Rhino as its almost invincible. Park in it in the garage opp to safehouse in the airstrip. Make a save to store in the garage. Now take the tank and drive it all the way down to four dragons casino entrance. From here on drive on the sideways and mow down anyone who walks on the path. Go all the way towards the end of the strip and return back the same way back to four dragons. You will net 150-250 kills based on the pedestrian spawn rate. By the time you reached the casino back there will 4-5 wanted stars. Just get down and save the game at four dragons casino to reset the wanted level and repeat the same process. 2000+ kills in about 20-30 minutes. For some reason the tank does take damage through police car explosions so have a watch. If it blows up, you may have to bring back the tank again from restricted area.
If you are lazy to bring the tank, any sturdy vehicle will do the job. Coach, slamvan etc.The game has tendency to spawn the vehicle you are currently driving, so if you use coach or slamvan instead of tank, by the time your car blows up there should be a same vehicle parked or moving around you.