If you did not unlock this trophy during the story campaign, a great place to do this is during Chapter 5 - London Underground. Use Chapter select to load it up on Very Easy difficulty. Note that this needs to be 30 continuous pistol kills in a row. If any of the enemies rush you, be sure not to kill them with a melee attack or counter. Just back off from them and kill them with your sidearm.
There are just enough enemies here to get this, so play through the chapter killing as many enemies as you can with your pistol. Try to kill enemies as quickly as possible since your teammates will rob you of some kills if you take too long about it. If you haven't gotten it by the time you get to the final door that you have to break open, then leave your partners to work on the door and head back around the corner. Several more enemies will come up the ramp at you. This is where I got the last couple of kills I needed.
Note: The trophy will unlock as soon as you get the 30th continuous kill.