Theoretically they shouldn't be able to outdo your indirect options if they're using so much manpower on bunkers.
AT guns also make very short work of them.
Other options include: Satchels and Demo Charges. I suppose you'd either use a scout car or smoke to get close.
The issue I run into is, when you have an AT gun try and destroy it, it just gets blasted by mortars. Not to mention you're either attack grounding or have to walk into its line of fire and be suppressed. And like I don't mean that the guy is only building bunkers, he has an actual composition behind it and in mid-lategame he's just spending his float on no popcap suppression platforms. It's just the amount of effort to break a multi bunker defense is ridiculous compared to how much effort is required to operate. Another problem is typically once that person gets their campfest setup, maybe some mines and some AT guns, they can then dump all their actual infantry and tanks to their teammates side and start doubling up.