As undoctrinal counters with Osthern, use smoke barrage from the mortar on the bofors and then rush a Flame ht, then attack ground the emplacement. After kill the bofors move your infantry to keep enemy units away while you finish the mortars with the flame ht , the mortar will still be counterbateried, but will buy enough time to kill the emplacement.
For more safety, pick a doctrine with panzer tactician so you can still save the flame ht if your mortar gets oneshotted. A not so easy but still reliable option to panzer tactician would be the smoke strafe.
Undoctrinally as Oberkomando west, you can force the brace on the bofors with a racketen and then trow grenades over it when the brace is about to finish. ISG are always an option, not the real counter but they will still help to kill emplacements that arent in mortar pit range by autoattacking, with help i dont mean on his own, but will still suport the previous explained tactic.
To help doctrinally, pick that bloody firestorm doctrine and burn those bastards down before they lay eggs. Aswell breakthrough helps a lot if the things go too mad and you need the Jagdtiger to kill everything, always supported by pzfuseliers spotting for it
Ever seen what happens to flame HT that go with smoke vs bofor's. A good opponent attack ground your flame HT and your HT goes down in 2s...