Board Thread:Questions and Answers/@comment-26257973-20150530215930/@comment-25209037-20150531041922

Unfortunately, elemental weakness damage bonus only apply to the unit's base element, it does not apply to elemental buffs and multi-element BB/SBB, an example of the latter is Rigness: his SBB deals fire/water/earth/thunder, but he will trigger only the weakness bonus against fire types since he is a water type.