There's several variables that will affect this, like the ATK of your primary eidolon, level of assault skill, whether you have elemental advantage, and current Grail buffs. For my Water team, I get more damage from running double elemental ATK, but I run with 1x Jorm because the benefit from the added HP and ATK -10% outweighs that. The difference in incoming damage going from -60% to -70% ATK is massive. At -70% ATK you're more or less invincible, unless the enemy buffs its ATK.