.. sigh the point is survival. A radiation pulse will hurt Harry the most, and will not be immediately obvious to people not fried by it. The idea is to disable opposition and summon help.

In the event no help instantly materializes, dodge and start the incantation for fiendfire - it doesn't actually matter if you can't cast it. Everyone present will recognize it, and to a blinded wizard, fleeing should be utterly reflexive at that point.