At the end of chapter 2, you will be given a very clear and unavoidable choice to go with either Judy (Resistance) or Devotion (Honour Grant). This will occur regardless of any decisions you've made previously in the game. The choice you make here is the only thing that determines whether you get this achievement or Hero of the Resistance
at the end of the game. Therefore, you should make an extra save here, so that once you beat the game with one side, you can simply load your save and just run through chapter 3 again with the other side, rather than having to replay the entire game.
When you've made your choice, all you have to do is keep playing the game until the end. Any choices you make in chapter 3 will have no impact on which ending you get, so you will always get the achievement tied to your chapter 2 choice.
If you have any questions, feel free to leave a comment and I'll try to help you