The Stalker-Pattern Bolter and the Chainsword are both obtained in Chapter 2. You must complete an entire Chapter firing only the Stalker and swinging only the Chainsword. A good place to attempt this would probably be during Chapter 3, as there are many groups of Shoota Boyz that attempt to engage you from above on catwalks.
Note: Throwing grenades will void this achievement.
To quickly knock this achievement out, load up the final checkpoint of Chapter 3 and grab the Stalker-pattern Bolter from the weapons cache. Now move on and finish the mission. After the cutscene where you and your allies destroy the cannon, simply move forward until Chapter 4 begins. Once Chapter 4 begins, bingo. Achievement Unlocked.