I just realized she now has lightning resistance and the others also got lightning resistance, so if my thoughts are correct if she gets res to any elements the others get it as well because she mama bird.
Back when they first met the humans, Amelia analysed one of Dere's feather, and it had lightning, immunity to Ice, Flame, and Slash Endurance. There was also a graph that showed 50% Ice, 50% fire, 90% insulation, and off-the-chart shielding. If we take insulation to mean lightning insulation. Then it would seem that generally they are just good against lighting. But those dragons are so powerful that they can overpower the insulation.
I think it just's shows how absurd the lightning dragons are that even with 90% damage resist it still murders them.
Also for reference, the 1st chapter says that 40% of monsters in the area they can defeat when working as a group. 60% of monsters they run away. So, presumably, they aren't #2 on the food chain. The lightning dragon just likes to eat them for some reason, most likely. Considering 60% of monsters they have to run away from, maybe they are just easy prey in the plains. Particularly as Reisu said, they just die in small groups. Without Reisu that 40% when attacking as a group is probably significantly smaller. They might be the bottom of the food chain without Reisu (With their main ability being they breed fast 3 years to maturity and eggs. They are most likely the equivalents of rabbits in the plains.