Pacifist mode is essentially hard and ultra hard modes. They require you finding passages and not killing most things or nothing at all.
But how you not kill the first boss?
(It's obvious you shall not kill the bosses.)
I imagine is all secret passages. No idea how you find them all. But given that only something like 0.3% across all platforms got the 100% pacifist achievement which requires you not killing anything at all. You can bet it is crazy hard to do.