tl;dr: Crimsonwhorl Bubbletear.
Having trouble staying alive after he shoots this homing orb at you once his hp reaches 50%? Dodge rolling at 180º keeps getting you killed by his other attacks? Even Brokenhound Step for some otherworldly reason can't save you? Fret not, you don't need to dodge it at all. You don't even need to have any sort of skill either. Just follow these steps:
Step 1: Run away once his HP reaches 50%.
Step 2: Count to around 5 seconds after he puts his balls out. (Ok, you need the skill to count. You did spec into it right?)
Step 3: Chug a Flask of Wondrous Physick with the Crimsonwhorl Bubbletear (converts all elemental damage into HP for 15s, the stars last for 20s).
Step 4: Tank everything he sends at you like a champ.
You'll live through it all without a single sweat!
Only works once though, so good luck if your damage is low like mine and he does it again later on... And Radagon's body that he keeps swinging isn't elemental, so you still need to dodge his sword attacks at close range.
If you're too close to him when he shoots it, chug before it hits you and hope you live the few seconds after the effect expires. It still worked for my dumb self: https://www.youtube.com/watch?v=F30kKx5pSdU
Currently, Hurray has 33,221 views for Elden Ring across 3 videos. Less than an hour worth of Elden Ring videos were uploaded to his channel, or 4.57% of the total watchable video on Hurray's YouTube channel.