Chilly Meridian, Burning Meridian, Drowning Meridian, and Tingling Meridian are abilities available to Oversouled Cactuars that decrease target's resistance to ice, fire, water, and lightning, respectively. Amorphous Gel and Protean Gel can use Barrier Shift to change their elemental weakness.
Ancient Magic, in addition to dealing elemental damage, renders the target weak against the weakness of the damaging element; for example Freeze (ice) would render the target weak vs. fire. Damaging Ninjutsu works the same way. Aside from player abilities, the enemy ability Nuclear Waste reduces all elemental resistances of player's hit by it.
The status ailment Imperil reduces all of target's elemental resistances by one class.
In Dun Scaith, the boss Ferdiad Hollow casts Debilitator, which either lowers the Fire or Water resistance of the entire alliance. Shortly after, he casts Flameflow, which deals either Fire or Water elemental damage to the party, depending on how they handled a certain mechanic between the Debilitator and Flameflow casts.
The status ailment Oil doubles fire damage on the afflicted target. It doesn't work in original release.
Oil increases the affected character's received fire damage.
Some Soul Breaks and Legend Materia effects can reduce targets' elemental resistances, along a sliding scale from -100% (absorbs the element) to 200% (takes double damage). Soul Breaks reduce resistance by 20%, while other effects reduce resistance by 10%. This can be stacked up to a 60% reduction in elemental resistance.
Players often refer to the effect as "Imperil <Element>".
Some abilities reduce targets' elemental resistances.