People think Serana is weak?! I tried to rush through the Dawnguard questline to get rid of her ASAP because depending on the type of character I was playing, she'd end up Boone-ing the whole room before I even got set up.
You said Frost Dragon, unless you meant what element it breathes.
I thought it was a Forst Dragon because I saw the breath, but it wasn't grey/silver. I didn't really look at it, but I'm pretty sure it was Elder, because it looked yellow or bronze.