In Killzone 3, in multiplayer, Brutal Melee an enemy player with the Silent Footsteps skill active. The Silent Footsteps skill is automatically unlocked when you achieve Rank 10 - Sergeant 1, which requires a total of 30,250 points. Equip this ability at your class selection screen under the "Skill" option and your footsteps will be silenced. It is recommended to use it in conjunction with the Marksman's Cloak or the Infiltrator's Disguise ability to sneak up on enemies and Brutal Melee kill them for the trophy.