i used to camping on a destroyed AA platform shooting every krauts at sight, the deadliest vehicles i encountered was loads of halftrack (and zeplins of cause).
IMO, if you stay in one place, no matter how many krauts you kill you'll still stuck at level 4.
but if you're more hit & run, the better chance bigger Nazis' vehicle to appear.
will try thanks EDIT: na still dont work, maybe because i m too early in the game?
As I played, when I got level 5 a larm, I was looking for some alone Nazi (or killed all Nazis nearby but one remained), punched and kicked him and got in disguised as a Nazi. Then I took any car and drove as fast as possible to get out of the alarm zone.