Using this method you will probably be able to kill the radiance on the first try if you know her moveset, so you should refrain from this unless you are very desperate or wan't to kill her in Steel Soul mode as it banishes all challenge the fight has to offer.

The point of this build will be focusing on your DPS. Now there is another guide using Abyss shriek, but to cast spells you need to gather soul, and as in this guide we will only use the nail, the fight will be a lot faster.

After challengeing The Radiance, quickly start spamming her face, keep jumping and hitting. You can pretty much ignore her attacks due to our large health pool, but don't get too greedy, if something is easy to dodge, don't waste your health.

We deal so much damage that she will skip the second half of Phase 1 and the platforming will begin immediately. Jump onto the lowest platform and the moment she appears, start spamming her again. By the time she disappears, she will be nearly dead, just follow her to the second spot and hit her a few times. The final platforming part will begin, which is hard to mess up. The fight will be trivial and she won't last a minute. (Actually it does, took me 1 minute and 26 seconds to kill her)

Good luck and don't forget to actually fight her before slaying her with this tactic.

Also, this build easily takes out every enemy in the game, except the Nightmare King, so if you are having trouble, use it anytime.