I would suggest that the best way to take them on, is to not allow the major country to build them in the first place (that is to be more aggressive in play style). Failing that there are some things you can do...
First of all, a hex is easier taken, when there are no forts or fortifications there. Since forts cannot be destroyed, then that is not an option with those. However it is an option with fortifications. The best option thus may be the use of mass artillery strikes. This has one unfortunate side-effect, bridges may get blown up (or most likely will). So what you need to do is be very careful how many artillery pieces you use for each bombardment. Artillery always does the same structural damage, regardless of how many attacks you split them up over. Bridges take a test for each attack you do. (structural damage d-side vs. bridges structural points d-side, the roll that is higher determines the result of the attack on the bridge: If you don't understand this in parenthesis don't worry it is just technical stuff).
The more heavy the artillery the more structural damage it will do. So Light Artillery would be worse at the job than Artillery. Artillery worse than Heavy Artillery, and Very Heavy Artillery would be better than Heavy Artillery. More modern artillery is better than less modern. This Artillery II is better than Artillery I.
Assaults can destroy fortifications too, but the defenders will get the bonuses that these fortifications confer. Another option is to attack from air as well. Bombers will do structural damage too. Just remember to escort them with fighters of your own, as they will surely get intercepted, if your enemy knows what he is doing.
Also I am assuming that these fortifications are across a river. In that case, it might be better to attack across a bridge. (Thus important not to destroy it). Some vehicles cannot cross rivers, and thus you would get less of a punch to your attack, if you need one.
All in all good luck... (ever considered going through Switzerland? Not that I think that is easy either).