For basic attacks they could employ the use of the "Elemental shots" Bolt, fire, and ice which could decrease resistances or to the elements they use (or even increase vulnerability to make it more technical). So fire shot could reduce ice resistance making it smart to use ice shot in succession with Fire shot. You could even use wind if you needed a fourth one.

After I think about it more though increasing vulnerability would probably work best. because BLM and ENG would have synergy. Adding a mechanic to the game similar to the way tech skills were used in FF11.