As soon as Grundy appears in the sewers beneath Industrial, there is a pool of water on the ground right in front of you (I would scan it before lowering the door that brings him out). If you have the Shock Bataraing, all you do is equip it, aim for the water, and as soon as starts to charge you, throw it. The water will electrify, bringing him down on all fours. Beatdown on his head, fight over. Didn't even break a sweat. Since you have to play through the game 3 times, I would recommend hunting down the Shock Batarang each time before confronting Grundy, It makes this fight so much easier.