Firstly, if you're unaware how to get a secondary fire/kill with the magnum, first shoot a bullet, which I would recommend firing next to an enemy, rather than at them due to that killing them with the normal bullet most of the time.
Also, a heads up, after you use the LARK (rocket launcher) the first time, you will come across a Bullseye mark II in a little pipe, swap the LARK for the Bullseye rather than the Magnum, because there'll be a bunch of enemies after the weapons location, giving you a bunch more kills if you still have the magnum/ammo.
Once you see an enemy and fire beside them, you'll see the bullet is in the ground - (You will see the glowing bullet there) - When you do, press
to make the bullet explode, killing any/all enemies nearby. Fortunately, there's more than enough enemies in the prologue for you to obtain this there, which is where I got this trophy.