For this achievement you must kill the commander of the tank and damage the viewfinder. When playing normally you should aim at tanks cupolas (the bump on top of a tanks turret) as this is where the commander and viewfinder are on MOST tanks. In addition you can use the methods below for better chances of critical damage to the commander/viewfinder:
1.) Use tanks that shoot primarily HE/HESH/HEP rounds. Some good tanks for this method are:
--Most artillery
--KV-2 (and gun but the ZIS gun)
--SU152 (152mm gun)
--M4 Sherman (with 105mm)
--T49 (especially the top package)
--and low tiers: M2 medium, Cruizer IV, T18, any other tank with a Howizter gun
2.) Use a rapid firing tank and a crew with the "Deadeye" skill. This skill increases the chance for critically damage components when NOT using HE/HESH/HEP rounds. This bonus is calculated per shot. Good tanks for this method are:
--Pz. II C
--T7 Combat Car
--T1E6-X(1)
--Any tank with a fast fire rate