Asking on behalf of the group's Wizard. He wants to electrocute the BBEG and have the spell deal extra damage through clever use of game mechanics. As a DM, I am all for it, as long as the rules somehow support it.
Create Water. You create up to 10 gallons of clean water within range in an open container. Alternatively, the water falls as rain in a 30-foot cube within range,* (PHB pg. 229)
Is this something you could cast, and either cast Lightning Bolt (8d6dmg) on the target being rained on(on your next turn / action), or use Shape Water to form a cube around the BBEG, and then Lightning Bolt to the face?
I personally think something being wet is conductive, which I might say means that the target being rained on would have disadvantage on the saving throw. My player really wants to try and argue that being wet yields a Damage Vulnerability (2x dmg), therefore causing lightning bolt to deal (8d6)*2 lightning damage.
Is there a rule I haven't found in the PHB about this already? I do really want the player to feel cool, especially since this upcoming week there is a BBEG confrontation and this might turn the tide for my group.