> Needs your empire to be pacifist (can't be done with Fanatic pacifist)
Step #1> Start in a galaxy with minimum size (Tiny : ~150 stars) et minimum habitable planet (x 0.25), with only 1 AI. Galaxy's form "Ring" may help.
> Step #2> Develop yourself better than your opponent
> Bring him a pacifist war (lol), and lieberate all his world at once (or in multiple time if you're not confident.
> Liberated empire will have your ethics : Pacifist + your choice.
Once the first AI has been defeated, only remains empires that have your ethic
> Congratulation, you're a true pacifist
That's all!
This guide is a quick and efficient way to obtain the achievement, but it can be completed through others means or during one of your casual games