My boy and I are on our second play though.
The first time we spent a metric FUCK load of time pillaging the swamp looking for iron because we were afraid of what enemies were in the mountains.
It turns out, all the mountain enemies are fairly easy to take on with maxed out copper armor, no need to dump all of that iron into armor.
We maxed out our iron maces, pick axes, and shields.
The pickaxes being maxed out are a great return on investment for mining speed and length of time before having to be repaired. You don't have to if you don't want to and don't mind repairing a lot more.
The iron mace can two hit wolves, and is insanely effective against stone golems, which we previously thought was only weak to pick axes.
If you simply pop bonemass before taking him on it'll be stupid easy. The maxed out shield helps turn you into a parry god and you'll hardly take damage against him.
Have at it, get your fuck load of silver before taking on Modder, and save all that iron for padded armor.