During the Undercity mission, you'll fall down into a pit with some rock pillars and the 'Rhino'. You can't defeat him by shooting. In the first part of the fight, just keep luring him to smash the rock pillars by charging at you until they're all gone, at which point he'll smash through a wall. Give chase, and in the next area once again lure him to smash a few rock pillars to clear the area a bit and give you room to maneuver. Then simply place a claymore and stand behind it so he runs over the claymore. A couple of these should finish him off and unlock your achievement.
Check out the video below for a full playthrough of the game showing all collectibles, miscellaneous achievements like this one, and the Moral Points I obtained (I skipped a lot of them) in order to get the Redemption achievement. The Undercity mission starts at timestamp 1:19:50