The Los Santos Slayer Trophy in Grand Theft Auto: San Andreas
The Los Santos Slayer
Reach a total kill count of 4,000.
How to unlock The Los Santos Slayer
This will likely be the last trophy that you will unlock. 4000 kills sounds like a lot, but depending on how aggressive and destructive you are, you should be quite close to 4000 kills by the end of the game.
You can grind out kills however you please, but I found the tank (Rhino) or the jet (Hydra) to be the most effective in earning a lot of kills in a short period of time. You can track how many kills you have by pressing and navigating to Stats > Crimes > People you've wasted.