I can't say why mediana took so much damage but it seems that with drain force regardless of the person receiving the damages resistances if their element is strong compared to the attack they will take less drain force damage and in your case orlandu [lighting is weak to kitones earth] he will take more damage. Values and stuff I don't know.
From what I can tell, when attacking someone with same defense and resistance, the other person with the weaker element will still take more damage and the person with the stronger will take less. There must be a hidden value saying that those with elemental advantage deal fixed percentage more damage regardless of all other stats.
the skill uses the units max hp not the enemies hp, elemental resistances do influence the amount of damage thats why its more effective on Orlandu, as for the damage to Mediena she probably has earth weakness so Im guessing shes using Ramuh Esper