I agree. It's hard to fight over land value. There are a handful of companies that do most of the assessing in NH and other NE states, so the procedures are pretty standard. If you file for an abatement and don't like the results you can appeal to the state bureau of land tax appeals. Again, pretty standard. Presumably, your due diligence and real estate professional found suitable comps that help form the basis of any negotiations. You can see the values of other properties on the town's database.
http://gilford.univers-clt.com/ Surrounding properties should have similar land values for similar size parcels. If it's shorefront, front footage is a big factor, not just total size.