So my group got to meet an enemy that can "only be hurt with magic". However, my party has no sorcerer or any other source of "magic" in terms of spells, power words etc. We have dwarf fighter, elf minstrel, half-elf rogue, human fighter and wolfkin hunter. However they have couple of magic artifact weapons (2x magic swords and magic great hammer). Since rule doesn't state that "Magic" means only spells I am gonna rule that magic weapons also can hurt such creature because those artifacts are full of magic and were crafted with it.
I am not sure what intend was behind "only damaged by magic" rule but since not every party is guarantee to have magic-users and magic is quite a wide definition in Forbidden Lands amd fantasy- I feel that it's fair.
I treat it simillar to "immune to damage by non-magical weapons" from DnD.