On my YouTube channel I've created a gameplay walkthrough video covering Chapter 3 of this game. I apologize for the audio quality. I'm still very new to creating YouTube content and failed to do a test recording to make sure the game could be clearly heard along with my voice. I made improvements over the audio from the previous two videos, but the settings still aren't where they need to be.
In this chapter we get the ability to extinguish flames making it a bit easier to take out guards going through crowds of rats with a torch. I can't remember any of these sections giving too much trouble. If you follow the video you should be able to get through without much fuss. If you struggle with the combat section I found it best to hang out in the corner. Keep your sling ready to fire and go for their heads. Hit the saltpeter bags to blind the shielded guys and get a shot at their heads.
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Note: As mentioned above in Chapter 3: Part 3 (A Burden of Blood: Rodric's tool from the chapter select menu) you gain the ability to craft Exstinguis which can be used to extinguish fires. To unlock the Night Guard achievement you need to extinguish 40 fires. This can be done through normal gameplay, but if you're in a hurry to unlock this achievement you can grind it out right after learning the alchemic recipe. I've made a guide showing how to do this, but in short you use the restart section option in the pause screen. In the video I made a mistake with the math (should have scripted before recording), but you can take out 2 fires from the location I show. With roughly a 15 second rotation you can clear about 8 fires per minute meaning you should only need 5 minutes to knock this out.