Use the bell in your key items to leave the castle. As for killing the boss, if you don't manage to do it with the strategy listed in the guide, here's an alternative one that's more or less just unga bunga. Requires some baby making to get enough points for learning skills.
A list of requirements:
Elucia skills learned - Hero Strike, Masochism, Resilience, and extra armament so she can equip double Nail Ornament, or double Sword Amulet.
Marie skills learned - Amplify, you don't need anything else.
Ran doesn't need anything.
As for setup of the actual strategy.
Turn 1 Marie and Ran need to go first. Ran uses a Euphoric Seed on Elucia. Marie uses Amplify on Elucia. Elucia uses Hero strike. Then for phase 2, Ran uses a single target momo to give Elucia 2 MP, Marie uses Amplify on Elucia again, and Elucia uses Hero Strike.
Results depend on how high you can get Elucia's attack.
Euphoric Seeds are sold at Altomyostr for 50 medals. If done properly, just 1 should be enough for both Phase 1 and 2 since it lasts 3 turns.
The strategy more or less focuses on just killing the boss in 1 hit, and since Elucia has a passive that prevents her from dying, she won't die on phase 2 despite taking a huge amount of damage from herself.
If you find you can't get your atk values up high enough with chroma alone, grind brothel more and unlock the stat level up in the church and raise atk for Elucia. Example below.