In addition to the aforementioned strategy, a good way to stop Ninth Dimension is to cast Berserk on the Trap Door. It will use its regular attack, inflicting around 1,000 damage, rather than Ninth Dimension. The Slow debuff will usually prevent the Trap Door from casting Ninth Dimension before it is felled by the party. The targeted party member can use a Light Curtain on themselves using the time between the target and the actual spell cast, which is basically the same strategy as having Rosa cast reflect.