The Invincible

I've seen a lot of guides saying that all you have to do to get one or the other is to drink or not drink the water.
That is not the case. It also depends on what you will say when Rohytra asks you what is your mission here.
If you admit to spying and drink the water you will unlock Beyond Divisions. If you refuse you will get On Her Own.
But if you say you are just scientist and you will continue to deny you're spying he will not believe you and you will get On Her Own even if you dring the water.

Remember to backup your save files after you start this conversation so you don't have to paly through the whole game again.

Here's my video guide:

There are 2 achievements in this part so after one of them you should be able to reload your save and do the other: Beyond divisions and On her own.

After following the convoy list you will eventually make your way back to the alliance field base and find the hideout in the rocks. You will then climb the rocks and Rohytra will capture you at the top and take you into the hideout.

Now that you're in the hideout proceed through the dialogue with Rohytraand when he asks why you came here or asks if you finally admit why you came here then this time select 'we were supposed to find out what you're here for', at this point you will go through dialogue and show him the slides of your team. He will once again offer you a drink and this time take the drink selecting 'Thank you that's what I need'. He will now trust you and after waking up and proceeding with the story you will gain this achievement.

At some point, you'll be offered a bottle of water. Take a sip of water to get the achievement. It will not pop right away, but shortly after as keep playing through the story.

Please take note that you can only get On her own or Beyond divisions in your current playthrough. You will have to start another playthrough for the other achievement.