This trophy can be obtained at any point, but would would be much easier to obtain if you are on the Easy difficulty as well as in an an area containing lots of enemies.
The former so you are less likely to be killed in the process, and the latter so you don't forget what your last kill was.
Your first opportunity for this is Chapter 6 - The Chateau
There is a large battle at the end of the chapter with plenty of enemies to kill.
The objective is simple, alternate back and forth between killing with any gun, and then kill with your melee attack. Keep doing this until your trophy pops