Destroying the two tanks is one of the side objectives. Those tanks arrive at the harbor when you trigger the alarm. If you haven't triggered the alarm and you are near the tunnel in the south-east part of the map (M4,3) then you can place two teller mines (sometimes only one can be enough). Alternatively, you can place a different charge near the mine (the other charge is detonated by the mine's explosion). This is the tunnel from where the first tank comes out after the alarm (a few soldiers will follow the tank).
The second effective option to destroy the tank is to use Panzerfaust again. If you aim with great accuracy, you can destroy it with one/two shots. But don't try to get closer to the tank - unlike armored vehicles that you fought with before, tanks have better range of sight, they can shoot explosive missiles and can notice you even if you approach them from behind.
The second tank can be easily destroyed from warehouse's balcony, in front of the armory (M4,2). This balcony is on the first floor and from there you can see a big part of the road on which the second tank rides. Panzerfaust can easily hit its target.