Is Serana supposed to be unhitable? When an enemy swings a greatsword at her it just goes clean through her body. She is totally oblivious to the enemy trying to attack her also. It's as if she was an object with no physics.

I know she is an essential character, but they can't have made her impossible to be hit, can they? Is anyone else experiencing this and is it a bug? It's really weird.

I had this happen too.
Finally it occurred to me to pop open the console, click on her and run the command "IsGhost".
Turns out something had set or failed to unset the ghost flag on her. Her read out from IsGhost was 1.

Remedy is click on her in the console then issue the command "SetGhost 0".

What if Joe is on xbox?

If on XBox, hopefully someone can craft a mod to fix it.

I wend digging through the quests and found the ghost flag being set there to "keep her hidden" until found when the Dawnguard sends you looking.
Sometimes it is not unset.
This is definately a bug.

Hopefullly some of the unofficial Skyrim Bugfixes are appearing on Skyrim Workshop, as it was intended to open use of mods to the console gamer.