Sniper Elite 4

At the beginning of each mission you can talk to various NPCs and/or use a radio to acquire secondary objectives; the radio missions must be completed in order to unlock this achievement.
In your mission selection summaries you can see which objectives you still need, you can start a mission on cadet, complete the objective, save and go for the next one. None of these are too hard and you can see markers for each objective so long as you use the radio prior to the mission.

There are 6 objectives that must be completed to receive the achievement:

Bitanti Village (2):
i: Signal Advantage – Find the radio operator before he calls for help
(I always found the radio operator within the castle, once I exited the village through a gate there was a small opening in the castle wall leading to a staircase and he always was there, kill him before he transmits)
ii: The Towering Assassin – Kill the sniper
(If you run along the coast of the island right of your landing point you will eventually see a ruined tower, the sniper is wearing camo, and is moving between openings along the upper parts of the tower, kill him)

Regilino Viaduct (1)
i: Hidden in Plane Sight – Find the downed American pilot; Find the crashed observation plane
(At ground level, below the viaduct, you can walk deeper into the countryside, near the rocky wall you will find the crashed plane, you will have to kill the guards and scuttle the plane. The pilot will found some distance from his plane, collect intel from his body)

Lorino Dockyard (1)
i: Hello to Arms – Raid the German armory
(In one of the large warehouses, with stairs and a second level, you will find the armory, take whatever you want and this will complete this objective)

Magazzeno Facility (1)
i: Naval Gazing – Destroy the German naval guns (5x); Assassinate the officer in charge of defenses
(There are 2 massive guns and 3 smaller guns, each must be destroyed with a satchel charge, does not matter if you are seen, just clear out your enemies makes this easier. The officer will be found on the farside of the map, inspecting a squad above a bunker, I personally liked to get a multi kill at this point, gives you a decent amount of points, 6 kills for the price of one!)

Giovi Fiori Mansion (1)
i: The Nazi Propaganda Machine – Find route of propaganda truck; Destroy truck
(Once you raid a small house on the left side of the map, near the road you will learn the convoy's route, set up your trap/ambush as you see fit and destroy the truck for this last objective)

Good luck and have fun!