As mentioned Interdiction and ground attack missions are able to reduce land fortifications (beachdefences can not be hit at all AFAIK), provided there is an enemy unit there.
CAS are best for the job with TAC being second in line. Strats perform very poorly in this role.
However it is considerable to run logistical strike missions at the same time to prevent the forts from being repaired after your strikes (or worse being repaired faster than you can hit them). For this the above ranking is inversed.