To unlock this achievement, "Bounty Hunter", you must complete all of Roland's jobs.
First off, you need to complete the mission that's called "Payback", then meet up with Uncle Po and Roland himself. Once that's done, Roland should be on your list of contacts on your cell phone.
You will need to do at least five Debt Collections for him, not including the one you did during the story. To do those missions, load up your contacts, on your cell phone, then call him and do what he asks. Once all five missions are completed, the achievement is yours for good!
Have fun and good luck!