First you need to unlock the Shock Batarang from Administration. You'll find this after following the Joker's trail for a while, directly after playing Match or Die.
Solomon Grundy is found in Industrial while you're chasing Black Mask. You'll end up in a long tunnel with electrical cables on the walls and some gas vents, along with a door around the middle that's locked by a terminal you need to hack. Hacking this terminal will trigger the Grundy fight.
Before he attacks you, make sure you've scanned the water on the ground near the exit door. As soon as Grundy bursts through the wall, arm the Shock Batarang and target the water on the ground. Throw the Batarang as Grundy steps in the water, and not only will the defeat Grundy the quick and easy way, you'll earn yourself this achievement too.