I started a NG+ And did it. The bad thing is that I'm stuck at capacity 440 but already had eaten all the mushrooms

Unless you have a save file in the main story from before facing her. Otherwise, you'll have to start the story over.

Time to restart from the begining. Or is there a way to continue from a certain point?

In that case, you have to feed her eggs in the story mode to see bloated killsha afterwards. It doesn't work in the rematch.

WHat I mean is that when I first defeated her in story mode, I didn't defeated her like that (feeding her eggs), but then, when she was at jack's resort you could fight against her on a rematch and I've defeated her several times by feeding her eggs but still nothing.

Thanks

I think you might be encountering a bug in the game. That situation works every time for me. Although, this video was from a past version of the game and I know it's been updated since then.

How can I achieve this (bloated killsha in jack's resort)? I've defeated her several times by feeding her eggs but still nothing (I didn't defeated her like that the first time, but I tried on several remaches and nothing.