When it comes to enemies dealing damage against you the player, elemental advantage/disadvantage is actually its own separate multiplier. The current formula for damage that players receive (not deal) is something like enemy_attack * (1 + atk_buff - atk_debuff) * weak_point_correction * (1 - damage_cut_total) / {10 * (defense_bonus_total) * (1 + def_buff - def_debuff + assist)}
I assume that the only source of defenses bonuses that aren't buffs or assists would be the passive bonuses for the soul?
And 'weak_point_correction' refers to elemental advantage/disadvantage.
As for the unregistered's question regarding pride event:
Assuming your grids are relatively evenly developed, ~12.5% more base atk shouldn't be enough to compensate for the lack of advantage. 238% (2x Jack o' Lantern + advantage) / 190% (2x Icarus/Adramelech) ~= 1.25, or ~25% more damage when looking at just elemental. Alternatively, 228% (2x Amphisbaena + advantage) / 190% ~= 1.2, or ~20% more damage.
Again, assuming the grids are relatively close, I'd just go with your dark team + 1x or 2x Amphisbaena. The real noticable difference here is how much less damage you'd take over time compared to your non-dark teams here.