This is story related and can't be missed.
The 5th war story is Laurence of Arabia. Your mission is to take down a armored train being used by the Ottoman Empire. The first mission is to take a code book from a derailed train. The second mission is to kill 3 commanders and send fake messages to the armored train to lure it into a trap. The 3rd mission is to secure a village and destroy the armored train.
The armored train can kill you quick so keep moving focus on the train with explosives you find in crates and the field guns. Don't fire more than once from a field gun before moving. If you find dynamite you can get close to the train to toss them on but be quick. Horses are an option too. Destroy the train and the achievement is yours.
Also please note you will have to play the first and second mission twice to earn
In the first mission although not impossible but it maybe difficult to find all the field manuals and stay undetected, again not impossible.
The second mission you will have to play twice because each codex requires a specific way to kill an officer.