You will find there is no optimal combo to use all the time. A lot depends on who you are facing, what type of equipment you have and how you play the battle.
I have been using Helen and Althea most of the time. I now use Billie quite heavily as well but trade off other maidens often to level them up for different strengths. I am trying to get Isana up to strength to help tackle Campaign 2 level 40. I am also leveling up Rox and Eli to see how they do.
Helen is at Lvl 78 but still takes a lot of damage from fire based characters. I assume she would do better against them with armor that is not weak with fire but I have not leveled up the alternate gear enough yet to try with.
All you can really do is look at who you are fighting and try to match who you fight with as best you can and use them carefully to make the best use of their strengths.