Formidable enemies that use powerful elemental magic attacks and have high resistance to all elements other than their weakness.
Raise your whole party's elemental resistances with the right equipment and abilities. While bomb juniors of each element can be damaged by non-elemental attacks to a certain amount, the mom bomb and dad bomb are both completely immune to non-elemental attacks.
However, they can still be damaged by elements other than their weakness, so try lowering their elemental resistances to deal more damage.