you should atleast invest into your fireteam, to counter wind, which is your weakness. If you mean with dual element that you use both, fire and dark in the same team, thats a bad idea. 1 off hime might be worth it to offset weaknesses or cover important debuffs, but having more than that will limit yourself because of how the weapon grids increase damage only of the same element units.

This is unrelated but i believe you should ask those questions in one of the threads already made, likely that this one might get locked.