Interesting, you mean drive in to range, fire and drive back right? Or using incendiary?
Often used start: gren, mg, gren, gren, T2, halftrack/pgen
Incendiary is great if you can afford it. Normal barrage will at least force the 120mm mortar to move. You probably won't be able to destroy or steal the 120mm, so make sure you keep your mortar HT alive to do counter-mortar work throughout the game!
Also, if your halftrack is un-upgraded, you can put your infantry inside of it and use it to capture points that are being hit by mortars. The HT will take damage, but you won't suffer manpower losses while capping.