Q: What element should I focus on?
The better answer is "don't". To maximize survivability against enemies of all elements you have to be flexible to exploit elemental advantage at best (deal 45% extra damage and reduce damage taken by 25%), or at least minimize elemental vulnerability (same numbers used against you.)

Ideally you would want a soul capable of wielding the best weapon for an element and a main eidolon of the same element to augment the percentage. Unfortunately you can't always get that; if your soul's best weapon in an element deals less than half of her best available weapon, then that +45% won't make up the difference. To complicate calculations even more, all the other weapons equipped in the same team slot contribute their attack and their elemental % bonuses, and these have to be taken into account when weighing whether it's better to equip a weak main weapon for its elemental advantage or a stronger weapon without it.

With the main eidolon you can be more flexible. A main with weak stats can be made up for with strong hp and attack values in the rest of the eidolon pool, and any of those eidolons can be summoned for their damage and benefits. Most important in a main eidolon are the % bonuses they provide each member of the team, as that makes the greater difference in survivability damage output.