On Chapter 3, you will find your second Purity Seal. This seal unlocks the 'Marksman Fury'. To use Marksman, click both thumbsticks, then pull and hold . When you do this, you will go into slow-mo and do increased damage. Now simply kill 10 enemies in one go. I recommend use the meltagun when you find it during Chapters 7 or 8, as it has a very wide area of effect. A nice spot for this is in Chapter 9, after your Valkyrie crashes and you have to start fighting off orks. The orks will come in waves at you, many including Gretchin, which makes it fairly easy to get 10 kills in one activation.
Also, make sure you do NOT let go of during this time. Doing so will end the Marksman Fury activation.