It doesnt do "basically no damage"
Ill find against a Dawnguard itll take like 8 or 9 blasts to kill one guy with Serana.
By myself I off him in 2.
Try playing on Master instead of Novice.

Without Serana, you do decent damage, with Serana you tickle enemies, we were ambushed east of dawnstar by a group of 4-5 bandits and 2 giants and a mammoth decided to join in as well... Decided to transform and when i was hitting the giants/mammoth with the drain spell it did barely any damage to them, if i had kept going i would have been fighting them for well over an hour.......