Elex

The three ending achievements in the game are unlocked when you speak with the final boss of the game, and are dependent on your "Cold" rating (see "Coldhearted" for how this rating works). You will want to play through the game and decrease your Cold as much as possible, aiming to be "Human" when you first encounter the final boss. There is a quick way to increase your Cold rating and make you less human, but no corresponding way to quickly decrease it unfortunately. That is why you have to play specifically human-like.

Before going to the final boss, make sure you have at least 1000 Natural Elex in your inventory. See "Beacon of Faith" for where to quickly purchase this. Eating this resource (by using it directly from your inventory) is a very quick way to increase your Cold rating, which lowers your humanity. So, save before the final boss while having the "Expressive" rating to unlock "Pure Rebel" and reload your save. Now you can consume 500 Natural Elex to change your rating to "Intuitive" which will unlock the "The Third Way" achievement, and finally you can reload that save once more and eat all 1000 Natural Elex which will change your rating to "Ice Cold" and grant you the "Beast of Xacor" achievement.