Mortar pit counter is more than obvious. Inf rush.
Bofors also isn't hard assuming you are ready to take some loses. If you aren't, you need to keep him constantly under mortar barrage from max distance. His repairing engineers will get wiped first, then it dies. Another option is flamethrower halftrack used trough shot-blockers via attack ground order (problem is that they can do the same)
17 pounder is, I assume, not worth mentioning.
Problem comes when he uses them both at once, then you gotta make two mortars (and bloody hell, dont place them on the same spot)with a halftrack for reinforcements, so they cant decrew them. Force bofors to brace first and stomp mortar pit with infantry (flamethrower equipped engineers are always great) and Pak gun, and smoke barrage the bofors even before he de-braces to get more time. I know that this is somehow complicated, but you can make it and they won't have any comeback chance if you wipe their emplacements. Good (for you) thing is that brace and stand-fast ability have a cooldown, while mortar attack ground order and auto-fire don't
