Rocket Genocider Achievement in Rocket League
Make 535 Shots on Goal
How to unlock Rocket Genocider
This will come naturally over time and is pretty self explanatory. To speed up the process, start an Exhibition match of 1v1 against no bots and just keep hitting the ball into the net off kickoff. You will get one shot on goal each time.
Note: It seems as if you can only get up to 2 shots on goal to count per kickoff.