How can one kill SU FHQ spam?

I have a replay but i don't think it's really necessary. So he rushed an engi to near our cut off, popped him in the building and the instant it was in, the building magically grew flags. So my response was to build mortars and spend first 60 munis on flames for pio. The first building was just too much, even after multiple attempts at flanking etc. His uber cons would shred anything that got near and once he got them to full reinforce hed go after the mortars. The mortars took like (two of them) 3ish minutes to knock down a building. Of course he did it in a good full health building. So ultimately we had to ignore the first one and push elsewhere as, we were stuck on our side of the map and the game was nearly over at minute 5. So we start to push and bla-dow, as soon as we ran into another soviet unit and he realized what we were doing, another building sprouted flags. now this one we actually didnt do too bad. Pinned the infantry with mgs and just shelled the crap out of the building, but its another 2 or so minutes lost. We advance and another 10 yards away we find a bunch of russkies and then he just made the nearest building another hq. I have no idea what to do against this strategy. If these FHQs are next to important points/pathways you can not ignore them. It takes too long and is usually very costly to destroy them, all the while ceding map control while trying to take it down. Ive tried a lot of stuff, from mortar HT incendiary, to luft supply off map incendiary, etc etc. This is a tough nut to crack, and used properly in team games i havent been able to beat it. i honestly fear urban defense on an urban map more than any other soviet strategy. as soon as you knock the pesky building down, there's another 5 yards behind it. okw mate going fast stuka is an answer but its far too late, just like 251 flames. it must be dealt with before T3 or your goose is cooked, but how?

Mortar/flamethrower are your best options as you tried. You may have gotten unlucky with your mortar hits or something - hard to say without a replay but just make sure you're using the barrage on the building on cooldown to maximize the amount of damage the building takes. The MHT is really your best option as 1 or 2 incendiary barrages should burn the building down completely. If you see the building is getting to low levels of HP grenades could be used to finish it off - but you mostly want to just indirect fire the structure and use the rest of your force elsewhere on the map if possible. They invest a lot into the FHQ and have major bonuses while fighting near it plus the ability to reinforce, if you can force their army to fight away from the building you mitigate their advantages while also opening up opportunities to take the building down.

Stuka comes at 7min and u think, it's too late?
Waiting for Urban Panzergrenadieres for effective counter for everything from SU

This commander have no late game which will render your opponents with shitty stock units if you manage to survive that long.

As Ostheer you can build a reinforcement bunker and attack them from long range. Pin down the infantry camping around the FHQ with an MG, use multiple mortars (more than two) to destroy the building itself. Also make sure to burn down every building you come across preemptively.

ok, basically what i already suspected. I didnt barrage with mortars so i'll do that next time. I just hate the accuracy of barrage and figured it would be better to attack ground through fow when los was lost. mortar hts have been the most effective and i usually have spearhead in my loadout. rngesus hates my incendiary barrages though, ask everybody with maxim spam ive tried to burn . the time i tried against building i had 2 mortars and a mht. it took the mht two incen barrages. i just think building three mortars or more is crazy, especially early. One needs some infantry as well.

The only issue i have with going elsewhere and leaving mortars to kill building is, what happens if they go after the mortars? If they have 2-3 units do it, you're losing one of the mortars even if you retreat the instant you see them. i suppose flamer pios defending mortars would mitigate, but i really like them with my army in an urban map. lots of opportunities for sneaky flanks with them. or leave an mg with them i guess.

I've only seen this done four or so times, im likely a lower end of mid level player, so guys like me dont do this often. although one time it was The Angry Bears (dammit napalm!!!) who did it to us, most of the time i don't think they were even arranged teams.

as for a seven minute stuka.... i rarely play okw, and my mates have never gotten one quite that fast. best ive seen was about 9-10. i don't doubt it though.

destroying every building on some maps would take 20 minutes. i have tried to level the buildings i would use for it and leave only the crappy ones that go down in a stiff wind if i suspect it though. cases that ive done that they dont use the FHQ aggressively though. but yes, when i see one, im afraid of strong buildings.

personally i think they should only be able to be built in connected territory like most other buildings, but that's just my two cents.

Mortar-halftrack with fire barage can ignite buildings and burn them to the ground.

This is what enemy used against my FHQ and it worked well.

This commander have no late game which will render your opponents with shitty stock units if you manage to survive that long.

This, also each one of those things is about 50 fuel which will further delay their teching

It's a "Win big early" strat; and one especially effective because it isn't seen very often.

I always make sure I have MHTs somewhere in my loadout, and should remember to check if the enemy have it in their loadout (sometimes I forget, and that goes badly...)

I know you said you're play OST, but don't assault nades or whatever the name is from special ops and salvage okw docs wreck buildings? They're only 10 muni and usually take out medium buildings in 1 go. Maybe this would work if your teammate could try one of those.

They will want to fight for the house they invested in. I put some MGs around to pin all the fighters who are reluctant to retreat. You will find the MGs work overtime and accumulate vet very quickly. Use incendiary rounds frequently. They will have a lot of reinforcements to pay for to keep up the fight.

As OKW -> Stuka zu Fuß and your opponent will play without a commander -> Lose as Sov.

As Ost -> At least one or two mortars or a mortar and a mortar HT if available. Don't send your units to fight near the FHQ. Try to hold them down with MGs if possible and bomb them with your mortars while your other units cap the other side of the map.