How to deal with M1919 and Defensive stance?

The real key here is not to target the rifleman, but to target the AT guns with your stuka. Then, if you can force repositions or destroy the crews, units like ostwinds, luchs, panthers, or kt's can come in and destroy the rifles, while infantry follow up behind to take territory.

Other ways to deal with the rifles are to spawn units (falls, jaegers) into buildings close to them and throw nades. MGs should also be able to out-damage LMG rifleman in a head to head fight.

One important thing to note is that in semois, the most important thing in game is to hold the center. Put a raketen in a house somewhere, get an mg or two to close down the center, wire off flanks, put a mine or two on the road and DO NOT LET GO.

Also, you MUST move up and take the positions that rifleman leave behind every time you force a retreat. That's critical to gaining and holding territory, and creating value from winning fights. You should be destroying sandbags, or at least wiring them off, putting down mines, etc. Entrench yourself.

I wouldn't recommend some of the suggestions here (ex: multiple sturms) because they bleed too hard. Optimally, you'd want to use vehicles, because they forces them to waste MP reinforcing, while you can just repair.

I can't really help out without a replay, and I play Semoisky with a very specific OKW strategy (take the flanks with 2 grens, with a sturm in the center, and then collapse in the center with a puma or flak + raketens/mgs).

I wouldn't recommend luft or fortifications against m1919's, rifles with LMG's can actually deploy def stance infront of a 34 and kill them due to superior DPS.

Falls are also just a MP drain against them. ISG is a good shout though

I think you missunderstood me: this is why i said "MG34s & ISG" - the 34 alone won't do any good, that's for sure.
Combine it with an ISG, and you cause a lot of bleed / retreat, increase it further by having your SPios around.
Keep in mind he was talking about Semoisky, so popping some Falls next to the suppressed stuff will certainly work in addition, and slow down the agression by his enemy.