An alternate way to do this for any of the characters' achievements exists if you feel it is better for you.
Take your high level 1st playthrough character, preferably no less than level 30, towards the end of the game's progress.
Choose Xbox Live from the main menu to search for co-op games. Make sure your high level character is selected. Now search for Custom Games. Find a game that looks like the players currently in it are in some action.
As soon as the game loads and you are waiting in the lobby, make sure you do NOT ready up yet. Go to Select Character, now choose the new character you need the achievement for and ready up to spawn in the game.
The way the game upscales xp will be so that a 1 or 2 kills your team mates do will knock you up quite a few levels in really quick. You will reach level 5 through the work of your team mates and unlock your ability.
Now take that newly ranked up character and start a single player game.
Kill all the bandits through the way to Zed's Clinic with your ability.
You should get this achievement once you start killing the 4 or so bandits that ambush you when claptrap gets shot through the gate and needs repairing.