How to deal with 120mm mortar on Faymonville?

Lately I have been getting completely destroyed by this mortar on that map almost every game.
Soviet player simply builds one or even two of these things, lets it sit in its base and the bleed is winning him the game.

And in the base it is basically impossible to kill.

Is there any way to deal with this?

The Leig can do a similar job, although i would recommend to get more units on the field and get superior mapcontrol.
The keyelement of the 120mm counterplay is obviously to stay mobile so i wouldn't recommend playing multiple hmgs or to camp buidlings.
I think Mechanized HQ tech with a fast Panzer II Ausführung L "Luchs" and depending on your mapcontrol Schwer HQ with a fast Panzer IV should be your best bet.
If you have troubles with ressources you will probably be forced into a command p5, else the Luftwaffe Ground Forces doctrine could be interesting as the superior speed of Valiant Assualt and the Smoke Bomb drop could be useful to flank enemy positions when they have fewer combat units on the field. Another choice if you want to play the Lategame would be the Fortifications doctrine which gives you axis to the leFH which will be hard to counter for the Soviet player as there is no doctrine with a heavy mortar and a offmap strike that can destroy your howitzer, so the heavy mortar and probably the rest of the army will get countered only by a matter of time.