LogFAQs > #961321703

I would do White Palace before doing Hollow Knight...no spoilers though.

Grimm and Nightmare King Grimm, while daunting at first, are actually some of the more fair bosses in the game because there isn't a lot of random stuff going on. When Grimm jumps, this attack comes out. When he starts on the ground, this attack comes out. Etc. There's no easy way around them, just have to practice.

For the white palace, definitely equip the charms that add soul when you get hit and the one that heals soul over time. There's no enemies, it's all platforming.