I started the game and tried being peaceful and getting the scenes of the first monster before proceding with the story, but now each time I encounter the Bargest it's an instant gameover. It says Terminal Corruption and I can't do anything to beat her. In Passive says I have Barghest Mating LvMAX
Can I befriend her so she doesn't attack me?
To befriend monsters you generally need to beat them and then let them go every time, do it at least a few times and then generally you unlock them once you move onto the next section.
As for the corruption, every time you lose to that monster you gain an additional level of corruption from lvl 1-5. At 4 there is a chance you start the battle in a very disadvantageous position, and at 5 there is a chance you just straight up lose or start extremely close to losing depending on the monster. Lvl 1-3 just reduces certain stars in battle with that monster. In my experience once you lose because of terminal corruption it falls back to lvl 4 corruption for some reason, so you have a better chance then. Switch to the easiest mode if you haven’t already, it is already pretty difficult in that mode and that is what the game is balanced for (or restart since you barely started).
Also, every time you win against a monster you reduce that specific corruption level by 1. Save scumming also doesn’t work because corruption is global iirc. This means that no matterwhat save you use, the corruption will affect that save even if it was before you got the corruption.