Here's a quick video showing how to get the Dead Quiet + Apocalypse Pacifist Achievements. What I recommend doing is loading up Act 1 Resurgence with a friend since it's a quick 2 minute level and quickly speedrun through it to the first safe room without triggering any hordes or birds and you can let the AI kill all the zombies for you and it won't void the Achievement if you're going for Apocalypse Pacifist. I also recommend using a character with a stamina boost to make running through the level faster and also be sure to buy some healing items.