the bofor and mortar emplacement have enough range to cover a significant portion of a 2v2 map. The mere fact that his side of map is unassailable is also a huge advantage. He and his partner are free to pressure you instead.
The only thing that can reliably counter advanced cancer is panzerfusilier combined with jagdtiger.
It is maybe because I'm use to play vs OKW or 2xOKW covering half of the map with their trucks, but there is always room to move. 1 Mortar to force him to brace, wait 20 sec and attack, I don't play Ostheer a lot but when I do, this tactic works everytime, if you can't take it before T3, build a stug, it is really potent vs it.
Emplacement are fine, same with the said OP commander, just focus the assembly point first. Only annoying ability is the counter barrage, it is maybe too much for the fortification commander. For sure do not invest too much in soft arty yourself to counter this commander, it doesn't work and make you lose.