Note: This achievement has to be completed in a single run, this means you cannot reload checkpoint, or quit to the menu and continue regardless of whether you or Valerie died or not. This means you cannot die yourself throughout the whole level either as this will count as Valerie dying as well. If you fail for whatever reason, you will have to redo the mission in chapter select.

This is achieved on Mission 6 (Treehouse). This mission lasts about 20 minutes and isn't too difficult so as long as you're able to kill everyone before proceeding so that Valerie isn't killed by surprise or ambushed. At the start of the mission, you should kill everyone at the outpost, stock up with ammo, armour etc. and then you will get into the jeep in the gunner seat while Valerie drives.

You should use the rockets on the jeep by pressing Down as they're a one hit kill towards jeeps and choppers, but aim with care as you only have 13 rockets before they run out. Destroy all jeeps and the chopper that spawns behind you then look forward and continue to kill mercenaries that are on foot.

Once you arrive at the large outpost, this area has about 15 enemies inside, you will want to enter from the back-right side of the building and slowly take everyone out from there, there are mercenaries scattered on the ground outside, inside the building, ground floor, and first floor as well as two watchtowers, so make sure to clear the whole outpost out before progressing.

When Valerie is in the control room, you will have to make your way to the building opposite to grab the keycard and nuke, at this point she will not follow you but instead stay in the control room, so as long as you have cleared the whole outpost beforehand, she is safe until you return.

Once you return with the items, Valerie will open the large gate where you should scan your enemies ahead and take out the enemies in the distance. A long range weapon is recommended such as the Sniper or the LMG+scope. Move forward, clear out the final two buildings, after which Valerie will get into a forklift and the mission will end.