As the title suggests, this trophy requires you to register 150 kills with your Pistol.
Most classes will attain this on their first playthrough due to the pistol being a staple of your arsenal the minute you step off the Normandy. If, however, you find yourself lacking the required kills for this achievement, there are a few options available to you to speed the process up:
1) Reload a previous save and find a mission that has a lot of enemies - the game registers kills outwith your save, so you can reload the mission over and over.
2) Create a new character, make a new save when you first exit the Normandy and play Eden Prime, reloading your save when you reach the Conduit (preferably utilising a Soldier for this method).
3) Mission: UNC: Lost Module - Visit the planet Eletania (Attican Beta Cluster/Hercules System). Upon entering the Hercules system Shepard will be given a mission by Admiral Hackett. When you touch down on the planet, investigate the downed probe (marked on your map) - this will cause several colonies of Pyjak ("Space Monkeys") to appear. Save your game, clear the colonies with your weapon of choice and reload. Repeat this until the achievement pops.
*Be aware that Eletania has a Level 1 Toxic Hazard - take necessary precaution!*