As I understand it, you'd have to make two Concentration checks. One to see if you can cast in the grapple (DC 20 + Spell level) and one to see if the damage throws you off (10 + damage + spell level).
Not sure what the 'spell level' of that ability is either. Does the description not say? Maybe it's an Su ability?
Not sure what the 'spell level' of that ability is either. Does the description not say? Maybe it's an Su ability?