The Skeletal Duelist carries both the Rapier and the Parrying Dagger, both of which deal piercing damage. Thus, it is advised to bring good pierce resistant armor such as Scale Mail and Scale Leggings to battle against them.
As well, Skeletal Duelists are weak to bludgeoning damage. Use it to trigger their demise.
Rapiers can roll extremely high, potentially up to 2d12 4x, with a max damage of 96. Tread lightly.