I remember The Hollow Knight portion being pretty trivial and quick after a couple attempts and I got the pattern down. I don't remember my exact setup but I'm pretty sure he gets shredded by one of the upgraded spells and leaves himself open reasonably often.
Actual final boss is pretty rough, though. And the DLC bosses were even harder, never beat them.
I was trying to beat Grimm first, gave up and the final boss seemed like a relief in comparison. I stopped playing after that. Didn't really want to deal with Grimm, and Godseeker seemed like it would be even harder.
IIRC the final boss' first or second phase is its most dangerous, so if you can survive its opening salvo it gets substantially more doable. Don't give up!
I was trying to beat Grimm first, gave up and the final boss seemed like a relief in comparison. I stopped playing after that. Didn't really want to deal with Grimm, and Godseeker seemed like it would be even harder.Grimm is actually super easy once you know his moves. Even the harder version of him.
Tips for Radiance
On the phase with platforms, just stand on the bottom middle one instead of jumping around. If he spawns close to you swing upwards to continually hit him. If he spawns away, just sit there and easily dodge his projectiles while waiting for him to come back. Gives you plenty of time to heal and dodge no problem. The final phase is ridiculously easy and you probably won't ever even get hit let alone die on it.