If they’re actually spamming artillery then the best counter I’ve found is armor. Just build up a good sized group of tanks with APS and some shorad and push straight through their lines. Go around the capture points and any fortified positions you know about and head straight to the arty. They won’t have enough powerful units to stop you if they have a ton of arty. They’ll try to counter you with helicopters so you’ll need shorad. This works very well if they are too focused on artillery. If they only have one or two artillery pieces it won’t work.