This achievement can be earned as early as the first battle in both Nightmare and Hundred Years War Campaigns. If you do not earn it then you will need to wait until you progress further into the campaign and unlock the ability to field two officers in a battle.
If you have started one campaign and are eager to earn this achievement simply load up the alternative campaign and this achievement will be unlocked in the first minute
To earn the achievement you must first form an army by pressing RT and A while two squads are stood next to each other. Once the army has been formed go and seek any enemy. Once an enemy is found again press the RT button and you will see various options; the A button will disperse your formed army and the B button gives the option to perform a mass attack.
Select Mass attack withRT and B and the achievement will immediately unlock.