Sim city is really annoying strat to play against, but its still very possible to counter. First of just being aggressive and not giving him opportunity to build up his city is very important, if not possible, then focusing down single emplacement he managed to build can help.
Also sim city can be largely avoided, unless map has 2 important points like 2 vps or fuel plus vp (which very few maps do) you can simply focus your efforts to other side of the map, leaving his emplacements useless and his smaller mobile army outnumbered.
Personally sim city is annoying because it slows down game to a crawl. even if he only secures one vp with it, match is going to take a good while while you try to dodge his arty and mortars.
I play 2v2's and most maps are small so they just built them where they could cover both vps and trust me when i say me and my mate got paks and arty and focused 1 of 3 emplacements but it still magically got back to full hp after barrages ended and had to wait for cd.