Leigs don't outrange the mortar pit.
Only with vet and barrage.
OP:
First of all, it's always better to save replays, specially when you have difficulty or are uncertain on how you could had won a game or done better. Words can't describe how you micro your units, how aggressive/passive you are playing, timing, teching, units built, etc.
I'll start by saying that real simcity is at least 3 buildings or a mortar pit + bofor which are built REALLY early on. 2 Mortar pits, while annoying, can't defend themselves against an "all in" attack. This is juts to make a difference against someone playing combined arms or just putting a single mortar pit (which is IMO a more effective way of playing). Not accounting for the situations Butcher is describing.
Stage A: early game pre simcity
The moment which a simcity player is the most vulnerable. With Advance Emplacement Regiment (AER from now on), he can skip building a RE and start deploying something immediately with IS but he still needs to float 400mp for a mortar pit or tech for the Bofor. If you play offensively, you would deny aggressive location of simcity or force him to change his plans.
Stage B: first building deployed
Generally it's a mortar pit. You want that thing to die fast. Either you commit to killing it or ignore it (go to stage C). Flamer Pio, MG, lava nades can kill or force brace. As i said before, you don't want to engage on a bleeding long fight most of the time. If the maps allow it, ignore it and just go for a 2v1 against his ally. Once you force one player out you can then proceed to swipe the UK player.
If it's a Bofor first, you can try using smoke (not to many players go full cancer and know how to attack ground) or kill it with indirect fire but been careful of the barrage (needs a forward assembly or units inside). Unless you are heavily bleeding the UK, a Bofor generally forces to ignore that zone or go for the long game.
Stage C: terminal cancer or full simcity.
What AER provides is an upgrade to make it more resilient to small arms fire, IS been able to build/repair (expensive bleed wise), forward assembly with engineer upgrade and a pseudo counter to indirect fire with base howitzers (doesn't work on small maps).
Once the simcity is completed, it will basically look like forward assembly with engineers, mortar pit/s and a bofor defending. Your focus is either the forward assembly or disabling the bofor.
As OH, you are basically relying on doctrinal choices or late game vehicles/tanks. The best/most effective non doctrinal options are early on with flamer pios, mortar (before forward assembly with repair or Bofor) and the flamer HT. Doctrinally you can use mortar HT, LeFH or any heavy offmap (if you are playing offmeta close air support, the AT strafe is REALLY good against it).
As OKW, you can try to shut it down early on with lava nades, Sturms and Luch. Once simcity is set and you can't approach it or you want to drag it for the long game, double ISG or Stuka (first you need to force a brace). Doctrine wise, Volley grenades or any heavy offmap can shut down a simcity.
The worst thing you can do at this point is trying to kill or decap anything with single units, staggering bleed and half committing to any attack. If you can't ignore it due to map layout, you have to be patient enough in order to get whatever units and resources you need and then make a full on attack. This means full useage of offmaps and/or smoke to disable automatic attack from the Bofor. If you can defend 1VP and practically 50% of the map you can delay the game enough in order for you to get whatever tools you need.
IB4 QQ: yes, mortar pit/simcity is stupid on how few micro it requires in order to be effective but on the same way it doesn't have any high skill ceiling on usage which means it's effectiveness is limited once you know how to play against it. Still annoying to play agains it.