"What amazing lightning!"
Translation:איזה ברק מדהים!
Are you asking because מדהים ends with ים? Or because "lightning" is general rather than a single lightning bolt? "What an amazing lightning bolt" is a good translation in my opinion. מדהים (if that's what you're asking) ends with ים because it comes from the root דהם, its plural form would be מדהימים.