Thing is the mortar emplacement will be under constant bombardment and in early game you don't have enough power to do important pushes to eliminate the threat. I'm talking mainly about ISGs here.
Related to AGs, I saw a live game yesterday in which one guy won against AGs and MG using a Wasp upgraded UC, which is a logical choice if we take into account that AGs have no anti vehicle snare. The ostheer player didn't build T1 though, so he had no faust. He built a Scout Car though, but didn't succeed to catch the UC, it eventualy died to an AT gun hit.
What happened to me is most of the time they made mortars/ISG my sniper was out and helped to clear the threat. Also i did not hesitate to crew the mortar with the engeener (when they finished repairing) to help winning the artillery war, i found that quite efficient.
About AG i think you are right. The wasp is the answer. The only time i tried to go for it (and remember saving munition) i was taken by surprised and my UC got destroyed by the MG. I ll do better next time.
Kozo.