At Guns take out schwerer HQ with attack ground 100 % you don't need a spotter.
You can use two combat engineers + a mortar. Or you can use a decoy tank and two engineers. So or so it's the T4 building and shouldn't be taken out easily when placed defensive. 120 munitions is a pretty cheap price to destroy that building, but you fail to see that.
You can use a sherman tank (smoke) + paratroopers, major artillery, assault engineers. It's easy to take out that building, therefore it was buffed after WFA was released.
The problem is that the vanilla factions have no teching features. USF get free squads, OKW get's utility buildings. Rather than nerfing the new factions, the old factions could need some love.
Using AT guns to take out shwerer panzer HQ with ground attack is very risky, and also very unreliable. Since alot of maps have alot of random terrain bumps and differences in height,unless you can see the shwerer panzer HQ, it will often hit the ground.
More so, obers will very easily wipe an at gun from the front due to AT gun cover being bugged right now, basically AT gun cover right now does not apply from the front, but only to the sides of the at gun.
Now of course, vet 1 USF at gun ability gives the AT gun 70 range and also 70 field of vision, while smart use of a t-70 + zis-3 can indeed destroy a shwerer panzer HQ. However, this is not easy to coordinate and like i said, can very easily backfire and is a huge risk.