You will need play the following sections in one go and will be given a silenced weapon which you should make full use of along with the Bursters. You CAN restart from last checkpoint in the event you are spotted, it will not disable the trophy. Basically you need to make your way to the Satellite Uplink undetected all the way. Chapter 4 “Six Months On” – Jungle Valley, follow the instructions of your partner as he guides you through the section and you should have no problems getting through the section undetected.
Chapter 4 “Six Months On” – Behind the Vale, in this section you’ll be operating alone. Move forwards until you see two soldiers split up. Melee the one left behind, move along and you’ll sneak up on another one. Melee him and look forwards. There will be a static guard on the left with his back to you, another in the middle on higher ground, a patrolling one moving in a circle clockwise around the area in the middle. Shoot the Burster when the patrolling guard passes it on the right side, shoot the middle guard in the head, then creep up on the static guard on the left. Go through the mini-passage on the right and crouch just before the exit; a guard is standing to your upper right on higher ground, pop his head.
In the next area, you can go left around a waterfall into the grass and melee kill the Helghan on the ground; then shoot the guard on the upper right. Stop at the metal barrier in front and left of you and look forwards to spot another guard in the cave.
Next you come upon two mechanical Crawlers and a bunch of guards. Don’t shoot them but instead crouch and stick to the grassy path against the right wall until you enter a dark cave and from here on there are no enemies.
Chapter 4 “Six Months On” – Cruiser Wreckage, a cutscene follows where an ISA soldier is executed. Kill the soldier before or after the cutscene, move right into a cave. Near the exit, wait for the guard nearby to move next to the grass and turn his back before you melee kill him, but be careful not to be spotted by the patrolling guard on the left. Pop that guard’s head and another patrolling guard should appear on your right. Head to the objective after that and a cutscene will play and the trophy will unlock after that.
The above excerpt was extracted from "Killzone 3 Trophy Guide" at PS3Trophies.Org.