In Killzone 3 multiplayer, get 15 kills using secondary weapons in a single match.
First you will need to spend at least 1 Unlock Point to unlock the first sidearm - Sta18 basic Handgun for the class you wish to play as. More powerful sidearms such as the M4 Revolver, VC8 Shotgun Pistol and the silenced M66 Machine Pistol and can be unlocked if you have sufficient points for them. It's best to attempt this while playing Warzone which will provide you with ample time to get the 15 kills.
The Infiltrator and Marksman are the best classes to do this because of their ability to Disguise or Cloak, especially when paired with the VC8 Shotgun Pistol in close quarters or the silenced M66 Machine Pistol from mid-range. Go up to 15 opponents and quickly kill / headshot them with your sidearm. You will be awarded 75 XP for 15 kills with sidearms and the trophy will unlock.
The list of weapons that can be used is as followed: