Select Offline>Campaign>Bots Off>Normal
Then select Chapter 1: Mission 1.
You will start off on the Lander turret and circle the area looking for a good LZ simply shoot all the bugs you can during the short route to your landing. I averaged roughly 40-55 kills per run. Once you land kill yourself quickly and retry until the achievement pops will take around 5 times.
TIP: Also shoot cars along the path to work towards that achievement.