Difficulty was designed with Manual mode in mind and thoroughly tested to use it in all difficulties.
It's designed as a "you should learn how to play the game more carefully and how to use game mechanics to your advantage". I know Cubone leader has already cleared HC and same goes for Ralts leader and partner without any form of save abuse so it's definitely doable.
As for the keep inventory password, it's not something that's all that necessary to give. It can be found fairly easily if you look into the menus too.
Groudon boss fight: just like all the boss fights, it has been tested to be doable with no items in Hardcore. It's a game of positions and it won't Precipice Blades unless it sees a target at Line of Sight. Also it takes place at a level you can get access to setup moves like Bulk Up, Cosmic Power, Calm Mind, Swords Dance, etc. It's more a game of preparation and less a game of bruteforcing your way.