Arty will do the trick sometimes, but it has to be heavy, and a lucky shot.
Other than that, your best bet is to get your infantry in cover and try to flank the thing. Engineers are best for this, since they have satchel charges, but zooks can also work. The best is getting them from the rear. Flamers are good, but very vulnerable.
Wooden bunkers are much easier to take out than pillboxes, besides, they only have a single MG. Sometimes, a 37mm gun or light mortar will even do the trick. Concrete will give you problems, so try heavy arty or flanking and getting in the rear with infantry (preferably w/ satchel charges), zooks, or flamethrowers.
If there is no cover (not even embankments to hide behind), then smoke the hell out of your approach to block LOS. MG Pillboxes have three MG42s and they will tear your boys to shreds. The 75s and 88s will do the same.
Tanks may work with lucky shots, best bet being the 105 Sherman or the AVRE (very short range though).
Overall, it is tough work for the engineers.