The following guide is for Ranger Mode (2 guns), attempting Shadow Ranger, Invisible Intruder, Invisible Man, Invisible Savior, Invisible Soldier and Rain Man achievements/trophies, and only using weapons found in the Metro, none that have been bought. The guide contains spoilers.
Kill the Watchmen with the Revolver and Valve.
Do not get the Duplet here, just stick with the Lolife and Ashot to kill the Watchmen .
Inside the flooded room to the left there is a Bastard. Swap it with the Tikhar.
Keep going. To the right there is a room (a Watchman will run into this room so look for that). Kill the Watchmen here with the Kalash and Bastard. Inside the room there is a Shambler. Backtrack to get the Tikhar.
Keep going. You will be attacked by Watchmen - kill them with Tikhar and Shambler, and wait for one to jump on Regina to knife it.
Near the end where you have to ring the bell for the boat there is a quad-barrel Duplet up stairs - use it to defend against the Nosalis while waiting for the boat.
Use grenades, incendiary grenades and Claymore Mines to kill the shrimp. Incendiary grenades will instantly kill any shrimp. The big Shrimp will take more than a grenade to take down so fire at it first and lay down Claymores while backpedalling.
Pick up the Kalash 2012 here in a house near the red missile.
Kill the Watchmen here with the Kalash 2012.
In a room where there are a few Nosalis eating the bodies, look up - there are Winged Nosalis hanging bat-like to the ceiling. One throwing knife to the torso should kill it silently.
Kill the Watchmen here with the Saiga and Helsing. Pick up the VSV near the ladder toward the end.
When you see the vision of being hit by a car, kill the Watchman with the VSV. Immediately walk to the left and into the house. There is a corpse clutching a Preved in the bath tub on the second floor. Kill the Watchmen here with the Preved.
As soon as Artyom has access to use the Gatling you must kill regular soldiers before the riot shield guard and flamethrower heavy, as they do not count as a kill. As soon as you kill one soldier with the Gatling, the achievement/trophy will unlock.