Got it !
I used the air mortar on Cape Helles, mode Operations, defending team,
and it worked like a charm
I think I had 13 kills in one game
In Operations, the enemy just bunches up toghether, as opposed to the other modes where everybody is all over the place
And then, it's easy to hit people. You can be just far enough, and behind a hill, only have luck no other mortar is trying the same.
Now, the gun, the Parabellum - I can see it's easier to take down a plane, compared with other guns, but it still is job. In fact, it's still very hard it seems, while planes are still overpowered two the second degree ..
The Lewis gun is not good for that either, but the M1917 I havent tried against planes I must say.