Then, do you have any tips for the mercy route, do i just let them flee or do i have to do something extra to that?
Yes, you always want them to flee. Never finish any of them off. But getting the mercy route also requires recruiting them. After completing the requirements, they should show up in My Room at the beginning of the next chapter. Here is how:
Chapter 1
Barghest
After blowing away the one that ambushes you with wind magic, treat her with a bandage. Complete area without killing anyone.
Lampas
After getting the last battery, the first Lampas you saw should be confronted by her boss, and set on fire. Choose to get rid of the flames.
Empuse
Spare her after she is defeated.
Iron Maiden
Automatically recruited after getting the other monsters in Chapter 1.
Chapter 2
Mother Rabbit
Read every note you see, before getting the cure. After dealing with the Haru and Liz, go back to the area with all the gravestones.
Haru and Liz
When they confront you, choose to lower your weapon to hear them out. They will ask you to buy lunchboxes. Purchase two of the most expensive ones. When you return, ask for a reward instead of the key.
Kazuya then gets cursed, and they will now demand drinks. Do NOT go back to the store. Go see Hyakki instead, who is now at the right of the store. After she removes the curse, fully heal and confront the catgirls again. After struggling, they back down and give you the key.
Muroko
After defeating her in the shrine, go to the area with the bridge and giant pit. There should be something called a "Human Key". Use it to open a secret passage in Akane's room, and get the memento about what happened to her brother. Finish the chapter, and play the memento with Dr. Yui's device.
After seeing it, you now have to tell Muroko what happened. To do this, on the Main Menu, choose Chapter Select, and pick 2-2. After defeating the Twins, you free Muroko.
Nina
Choose to fight her by dancing. You need the ratings to be at 90% approval before its her turn to dance. Then defeat her. She gets killed off by M.O.W if you did it right.
Chapter 3
Echo
Read all four notes about the Echos. Once you shut off all the water pumps, the last one has a man's body, with a bag of powder by him. Interact with the bag, but do NOT pour it. Echo confronts you, but you have to put away your weapon and use the Persuade option to stop her.
Sphinx
Just solve her riddles.
Arachne
In order to recruit her, you need to have bonded (Affection 100) with Empuse and Lampas BEFORE you end up in the nest. Spare her after winning the fight.
Titanboa
After leaving Arachne's nest, Hyakki kills Titanboa and herself in order to save you. There should be a note left behind, saying that if you want a better outcome, go back in time and buy her secret weapon. After reading this, save at the tent and go to the main menu.
From here, go to chapter select and choose 3-1, the one with the Echos. After buying Hyakki's bomb, playthrough the chapters normally. Next time you see Titanboa, Hyakki makes a wager with her. Beat Titanboa, and she should be recruited.
Chapter 4
Camazotz
Just spare all the ones you fight.
Daji
During the second fight, there is a cannon you need to charge in order to beat her. Once its fully charged, fire it. You should be interrupted by your fox friend, who pleads for her life. Spare her.
Hyakki
Recruited after getting the other girls and clearing the mercy route.
That should be it.