Let's see...

Initial damage:
Two balls, five rounds... 4d6 per round, 20d6 total, with 10 reflex saves to reduce damage by 2d6 each.

The spell may not be too overpowered (!?), if:
- You're already desperate for move actions.
- You frequently fight things that is good at reflex saves.
- Everything you meet normally dies in less than 3 rounds anyway.