You'll have to avoid the PLR crawling around the place for this mission. Crouch and stick in the shadows. Follow the straightforward path until you reach a broken sewer pipe. Crawl inside and continue to the next building.
Once there, carefully and stealthily approach the nearest enemy and stab him from behind. Take cover and take out the other two guards. Take out the other guards in along the next corridor as well. Grab another weapon so you can have something to switch to then head outside.
Take out the guards and continue to the objective marker. There's an ammo crate nearby so use it to fully restore your ammo and grenades. You need to clear the next area of enemies. There is an achievement to be unlocked here so carefully approach the hanging car and press B to kick and drop it to the enemies.
Two for the road!
Head inside the garage and resupply using the ammo crate in the corner. The rear door will open. Hide behind a cover and take out the enemies waiting outside. Continue inside the wrecked bus and a rocket will blow it open. Before heading outside the bus, there's another achievement to be unlocked and its a little bit tricky. First take out the three enemies nearby. There are two enemies that will appear as soon as you exit the bus. The building beside them will crumble and you have 3-4 seconds to kill them both before the debris buries them alive. See the video below.
Leaving them alone could've saved me a few bullets...
This is easier done in Easy Mode since you can aim faster and more accurately, and enemies die faster. You just need to sprint outside and immediately zoom to auto-target the two enemies. It is recommended to kill the enemy to the left since he's closest to the crumbling building.
Once done, head to the next marker and kill two more enemies inside. Breach the door below and continue along the corridor to finally meet up with up with Montes. Follow him until you regroup with your other surviving allies. Run to the marked Humvee and man the turret. Use it to fend off the attacking enemies. More enemies will appear to the right. Rain lead to them until you get the prompt to run to the Osprey. Mission complete.