The Conscientious Objector achievement is earned by playing from Home Castle to Barbarian Boss without attacking any enemies.
Considering that you can't advance each battle until all enemies are killed (the entire point of, you know, battling), does that make it impossible to achieve without playing with friends? Or can you rely on damage-dealing pets to attack enemies for you as an exception to the achievement requirement?
Best Answer
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.