The description specifies 'with heavy weapons', but all three guns Hans uses: the Hellbreath; the Gatling; the Medved - count as kills. The tank itself does not count as a kill (though the soldiers on it do) and the enemies continually running up the right side of the bridge do not either; only soldiers in the central bridge count. It is advised to take out as many snipers as you can, and delay killing the Gatling-gunner to kill more snipers. Use the Gatling on everyone else. If done correctly, this should unlock during the final wave, before the tank.