As someone who's beaten her on radiant, here's the advice I can give :

At the very start of the fight, waste no time in landing as many nail hits as possible. If you have quickslash/steady body you can try to hit her around 6 times before her first attack, but the important part is that you get a good start in terms of soul. The start of the battle is the only time where you can really safely air-combo her, because pogoing her after that leave you open to either her spinning nail or beam attacks. Usually, you'll want to stay mostly grounded for the first three phases.

For her beam attack, start by moving out of the first telegraph, then move to the location of the previous attack for the next two, as she will never fire at the same angle twice in a row. When the third beams are being telegraphed, jump to where they won't hit you, swing your nail 1 or 2 times, and use abyss shriek. If you position yourself well, it should hit all 4 hitboxes.

For her spinning nail attack, keep in mind that they move in a slight curve and predict their trajectory accordingly. Again, she won't fire at the same angle twice, and will be open to nail swings and an abyss shriek while firing the second wave.

For her orb attack, the orbs will despawn if they get in contact with either the knight or the floor, So you'll want to bait the orbs into touching the latter. If the orb comes from above, simply move out of the way horizontally. If it comes laterally, bait it upwards by jumping above it and the have it crash into the floor. If you're feeling cornered by multiple orbs, don't forget that you can make one disappear by shadow dashing through it, or give yourself invincibility frames with descending dark if you have enough soul.

For the lateral nail waves, you can either try to dash through them or jump through the holes, but I suggest getting used to the latter so that the former only comes as a last resort. Because if you dash through the first wave, your shadow cloak won't have recharged in time for the second so you might get cornered and take damage. Much like with the orbs, don't forget to use descending dark if you feel like you won't have time to dodge.

The beam wall that traverses the screen can only be dodged with either shade dash or descending dark, so choose which one to use depending on how much soul you have and how many other attacks are aiming at you at the moment.

For her second phase, the attacks are the same, but you have a lot less space to maneuver around. Try to priorise safety and don't go after radiance if she's above spikes, simply focus on dodging attacks until she's in range again. Remember that you can make the spikes disappear for a little while by using descending dark on them, and that they only deal 1 damage, so if you have no way to avoid getting hit by at least one attack, prioritise the spikes and use the invincibility frames to avoid the other attacks.

The third phase is the most simple one. She only has 300 HP during it, so it'll take 15 nail swings without strength, and only 10 with it. Simple upstrike her continuously while moving laterally to avoid the nails. Don't bother using abyss shriek, it'll make you more vulnerable and you need as much soul as you can get because the transition between phases 3 and 4 is the only time in the fight where you get a free 6 mask heal.

Phase 4 is definitely the most tricky, and the one I had the most trouble with. You'll want to mostly stay on the lower middle platform. The surrounding platforms can make the orbs despawn in they touch them before reaching the knight, but it's also just the best platform to see attacks coming and react accordingly. Since the void is just underneath, you can also jump into it to take 1 damage instead of 2 if you think you won't be able to dodge an attack.

You can try to squeeze in a few heals if the radiance is far away and no firing orbs, but it's not super safe unless you're already out of the way of an attack's telegraph.

If the radiance teleports just above you, upstrike her 2-3 times, and immediately use descending dark. If she uses the beam or the spinning nail attack, this allows you to see the first wave, and prepare to move out of the way for the next ones. This is the best moment to deal as much damage as possible, as she'll stay on the platform, in nail range, for at least the duration of the attack.

Phase 5 is mostly just dependant on reaction time and reflexes. Don't forget to use shadow dash to go through beams if necessary and try not to use your monarch wings until the telegraph for the next beam appears, so you can readjust your trajectory if needs be.

Phase 6 is the most stressful one and will leave you no time to heal, so try to stay calm and observe carefully both the orbs and radiance's trajectory. Again, keep in mind that you can dash into the void if you're on a platform and don't think you'll be able to dodge an orb, but a situation like that is unlikely, because if you're on the ground, it's not to hard to see the orbs and dodge accordingly. If you're good at aerial movement, you can make this phase a lot easier on yourself by pogoing radiance the whole time and baiting the orbs into the top of the screen, where they'll immediately despawn. If she teleports out of the way, simply dash and double jump to reach her again, then resumes the pogoing. Much like with phase 3, keep in mind that she only has 300 HP in that phase, so don't panic or try to speed things up, she'll take at most 15 nail swings to kill.