I already tried entering the hotel with 0, 100, 101, -1, -31 etc, nothing makes the scene appear, so i got feed up and messed with the files, got the true ending now. I'm not entirely satisfied, but will do.
I'm following Hahaha's guide, for me it's the fastest one specially if you already finished the game once. I didn't know that Jack killed any character other than Marie, the one boss who i know that kills a lot of npc and heroines is Jubjub, but that's only before fog10 so you have time a lot of time to kill her.
If I remember correctly, Jack only affects Marie. You could get rid of him as early as you possibly can, and those extra NPCs still won't survive.
IIRC you can find all the books you're missing from these places.
Levels have diminishing returns, and you are FAR past the point where you should be buying colored souls (preferably black from Hein at Hanover Abandoned Station if you haven't killed him) instead of leveling up. DLC1 content (Ship's Graveyard and Deep Sea) is noticeably more difficult than the main game.
Try using your break when they do special attacks, and remember that you can use infinite healing items every turn, so full heal every time. I used the deep sea knights weapon in the late game, imo the best weapon paired with some white rabbit rings or rings that up you attack power. Also the enemy you mention can be safely avoided if I remember correctly.
I'm doing that, but it's not stopping him. It goes like this:
Barber's Ring prevents debuffs, not status ailments (states). None of Blackbeard's or his ship's attacks are breakable. I'm too lazy to write up a guide in the proper format for him, but here you go.
google translate is a bit shitty but i think you know what the exact meaning is
