Missable. During the story mission, Madd Dogg’s Rhymes, you will infiltrate Madd Dogg’s mansion and have to steal his lyrics for OG Loc. It is during this mission that you will be introduced to the stealth mechanic. Basically, you push in the left analog to crouch, sneak up behind an NPC that hasn’t seen you while equipping a melee weapon, target them and attack. If done right, you will instantly kill the enemy. In order to get this trophy/achievement, you will need to stealth kill all of the enemies during the mission. That means if you are seen at all, then you will have to either replay the game up to this mission, or reset an earlier save.
The first couple of stealth kills in the mansion are given to you through some tutorials, but after that, you will be on your own. Once you reach the bar area, you should ignore the two enemies in there and continue to the next area. There will be another guard here, right before being able to grab the rhymes, but you can get the jump on him by sneaking through the room with the armor pickup in it. As soon as you grab the rhymes, some more enemies will spawn inside the mansion as you retrace your steps to escape. At this point, hide in the nearby dark room and wait for the one guard to come through, before stealth killing him. He should drop a Silenced Pistol, which you can use to kill the rest of the enemies with headshots. Be mindful of the two enemies moving about after the bar area, as you don’t want them to see you. The trophy/achievement will pop after leaving the mansion, should none of the guards saw you.