Magic Missile, Mana Bomb, Prism Trap and Switch Strike all don’t have a elemental type yet. They’re the perfect candidates to have Arcane.
If dark was a monster tag, then I’d make Arcane effective against Holy, Dark and Astral; but at the moment it isn’t a monster tag so I’ll stick with Undead.
(also Gnarling Snowman can be ice type so it could become a monster tag!)