Go for same elements. The way this game scales your damage, is through your weapon grids, the weapons have a skill called Assault which gives % increase damage to the same element as the weapon. So you want a team, all with the same Element ideally (1 off can be ok if it is really worth while), a full 10 weapons of the same element, to stack as much damage and HP as possible through this Grid.

If you use several different element himes, you will have the problem that you can not strengthen them equally.