Hell Let Loose

From the Main Menu, Select Basic Training and then select Practice Range

Once in Press to create an Infantry Squad.
Select Officer, spawn in on any spawn point and then pause and click Redeploy.
After the 3 second call down, press to change your role and select Rifleman.

Repeat the process for all the Infantry units.
(Officer, Rifleman, Assault, Automatic Rifleman, Medic, Support, Machine Gunner, Anti-tank & Engineer)

Once you've gone through all infantry, Redeploy and press to leave the infantry unit.
Press and select the Armour unit and select Tank Commander, spawn in and redeploy as a Crewman

As above, leave the armour unit and select the Recon squad and spawn in as a Spotter. Redeploy and spawn in as a Sniper and the achievement will unlock.