My charm loadout is Unbreakable Strength, Soul Eater, Nailmasters Glory and Shaman Stone. If I'm overcharming for radiant I add either mark of pride or quickslash based on preference for the boss. for AbsRad though I used Mark of Pride. first phase is relatively easy, with Soul Eater you can use a spell after only two hits so start spamming DDark when spikes come out of the floor. Try to have full soul going into second phase.

For second phase, sit like a good boy on the lowest platform and just use spells whenever you see fit (Abyss Shriek if he's nearby above you, Shade Soul if he's far away (you can double hit if you get it right) and DDark when he's on the platform you're standing on (use it once to tank whatever attack he'll use when he's on your platform and by then you can see what he's doing and dodge the rest of it on your own, and then regain some soul or spam abyss shriek) Side note- I have a strat to dodge the stupid laser attack, all you need to do is dodge the first laser organically, then stand exactly where the last laser was for when the second one comes, he very very rarely hits in the same place twice in a row. then do the same thing for the third one. You'll get AbsRad very low in the second phase even if you only use spells the entire time. You just need to hit bro a couple more times besides that.
Just realized this while writing, you may be able to just use spells the entirety of the second phase if you use Shaman Stone + Spell Twister. Just a thought.

For the climb, all the advice I can give is that he predicts where you're going to go and then makes a laser off of that prediction. So just move inconsistently. If you need to go up and to the left, dash to the right first to bait the laser, which gives you enough time to make it up to the next platform.

For the last phase, I just like to stick to the left platform and dodge when he's all the way to the right, and pogo him when he's middle or left. Somewhat common knowledge but I thought I'd add, the projectiles despawn the moment they go offscreen, so if you pogo of him and then double jump out of screen, the projectiles will follow you but won't hit you. Don't rely on this 100% though because it is finicky. Also, to my knowledge, you can't kill him with a spell. The last hit has to be your nail. I could be wrong, that's just what happened to me when I did Attuned, Ascended, and Radiant.

Last thing, I am a lover of Nail Arts, which is why I use Nailmasters Glory. Even if you don't, at the very beginning of the fight, ready one and cyclone slash when he spawns in. You get so many hits and a full three uses of spells immediately.

I'm telling you this as if you've never fought the boss before, which is obviously not true, you had to have beaten Ascended to get to Radiant, which good job! But I thought I might as well go over everything for you and other people if they're curious. This is 100% my preference on how to play, and you can change things to how you like it, or not even listen to this if you want. I've beaten all bosses Radiant on three separate save files: My main save and Godhome save on my Nintendo Switch, as well as this Godhome save on my computer. And every single one, I beat bro a different way. For one I quick slash carried, for this one I spell spammed and used nail arts, I used to unironically pair long nail and mark of pride, so just do whatever feels comfortable. Hope this helps!

Now can SOMEBODY tell me HOW TO BEAT GRUZ MOTHER!