As the diagram below shows, Venomancers' and Mystics' Wood damage is strong against Earth-type enemies and weak against Wood type enemies. Seekers' Archers' and Clerics' Metal Damage is generally strong against Wood-type enemies and weak against metal.
Earth > Water > Fire > Metal > Wood > Earth etc
Game abbreviations : ER > WT > FR > MT > WD > ER
Wizards can do Fire, Earth, and Water-typed damage (and have shields that protect against those types), and Psychics' spells are the Earth and Water elements. Stormbringers have access to both Water- and Metal-typed damage.
Some melee classes have abilities that do elemental, magical type damage, but for the most part their damage is Physical, rather than magical.
It is empirically observable that many if not most armor loot drops are lacking one or more of the magical resistances, and that the missing element, or among the missing elements, is overwhelmingly likely to be Earth. This makes Earth-type bosses such as Qingzi, Fushma, Fugma and Gluttonix much more difficult for low level and impoverished players who rely on drops. It also is an advantage for the strategy of always selling these drops, to use the money to buy from vendors if there are no good player-made armors or Quest Gear available.
Clearly, even after the release of many expansions, there is a relative shortage of Fire-typed damage available to squads, and three classes whose damage is weak to Metal, making Metal-typed enemies slightly tougher to take down, at least until a new class is released that does Fire damage.