I've had vickers supress a squad and then it crawled behind green cover and lost the supressed status while the vickers was firing on the squad the whole time. I have no idea what's wrong with that hmg
Easiest way is just to break their line since they invest a ton of manpower into emplacements which means they have no manpower for anything and then shrecks + inc nades do the job.
If you can't, go for a stuka or ISGs. 2 Isgs beat a mortar emplacement without problems. If you get a stuka, try to force brace with your teammate and barrage when it ends. Incredibly easy veterancy for the stuka and at vet 4 it's just ez pz if anything survived with the flame barrage.