For this one you'll find the guy in Thousand Cuts, he'll have the exclamation mark over his head and right beside a cliff after you pass the bridge. You'll know its the right bridge when it says you found that location.
Basically you have to shoot him in the head, not the arm, not the torso, not his legs, in his head. When you do that, achievement will unlock. If in co-op if your co-op partner shoots him in the head, you'll get the achievement as well.
You can go for this achievement during the mission The Once And Future Slab
Sorry was tired when I made this, shall I walk you through how to get there now? You'll go from The Highlands to Thousand Cuts, to get to The Highlands you go from The Dust and so on and so...forth/backwards, whatever. Anyways, after you enter Thousand cuts continue straight until you can turn right, there should be vending machines in a building where you made your turn.Continue straight around a building in the middle (if you'll lucky you should see him, he's a psycho) and cross a bridge, you should have found the location Slab Town, if not, you're lost and should just shoot randomly in the air until the non-existent police show up.
After entering Slab Town continue straight and you'll have to make a left turn and you'll see a bridge, that is called Broke Face Bridge and you'll want to cross that. Now when you get off the Bridge take an immediate left and follow along the edge of the cliff and there is a slight...drop(?) and he is in that area, jump down or walk down, whatever you want to do, get the quest, pretend he's your brotha and pop a cap in his face. And BAM, achievement unlocked.