What are Lightning Elemental enemies weak to?
As far as I have seen from looking around online it appears as though lightning is not inherently weak to anything, nor is it stronger vs any particular type.
This article confirms what you said about ice vs fire match ups, along with their bonuses to general enemies. For lightning weapons however, they simply mention:
"Lightning - based weapons will shock foes, leaving them momentarily stunned. They can also cause enemies to drop equipment, making them easier to kill and also bulk up your arsenal."
There is no mention of bonuses vs other elemental types, or any mention of lightning weaknesses. So I think it's safe to assume you are right in saying they simply take regular damage from everything.