Just build a single mortar or LeFH, literally, thats all you have to do.
You realize bracing and repairs are a thing?
A single mortar is never enough. The mortar halftrack is good, but as mentioned above, LeiH is the best, and now that it's usable in 1vs1 after the cost reduction, it's definitely the best option.
In either case, however, DEFEND your artillery. No matter how tempting it may be to advance, keep an MG crew near your artillery or a Panzergrenadier squad hidden on your side of a building near it to kill any infantry that tries rushing your artillery.
If the enemy rushes your artillery, you get free kills. If they don't, you demolish their emplacement eventually. Temporarily losing a bit of territory on the fringes, if it comes to that, will be worth it to preserve your artillery while taking out the enemy's stationary powerhouse.