Is it possible to achieve Conscentious Objector as a solo player?
After some research (playing a lot) I came to the conclusion that your theory will not work, because the damage dealt by pets is counted as damage done by you.
A way to earn this achievement solo is by using two controllers. Just kill everyone with your 1st controller and only move with the 2nd controller.
There are more trophies/achievements that will require more than one player.
More info on a wiki.