This trophy requires you to register 150 kills with your Shotgun.
If you near the end of the game and find that you don't have this achievement, there are several methods in which you can attain it without another playthrough, some of which are listed below:
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!*