Once free, Castiel goes for the appendage that's holding Donna again, stabbing and slicing at it in hopes that it will loosen its grip long enough for Castiel to pull her free. If he can get her out, they'll have a decent shot at getting away, but he needs to get it to react to his barrage of attacks first.

He jabs it hard and grabs for Donna's wrist in hopes that it will slacken.