There are four secondary objectives to complete in order to unlock this achievement.
Regilino Viaduct (1)
The Big Bang Theory – Locate and destroy the additional ammo dump; Find the ammo cache and destroy it
Abrunza Monastery (1)
Annihilate the Artillery – Destroy the artillery guns (3x)
Magazzeno Facility (2)
Crack the Code-Breakers – Acquire intercept digest documents (2x)
Enigma Variations – Wreck the code-breaking center