ELDEN RING: How to Kill Radhan Easily

Use Ekzyke’s decay twice, once in the first phase and once in the second phase. Just stand back and let the rot do it’s part. Once I found this out he was super easy to kill. The rest of the guide deals with this in detail. But the core of the guide is in this first paragraph for easy access :).

Ekzyke’s decay can be bought for two dragon hearts in the cathedral of dragon communion in caelid. However, in order to purchase this incantation you must first kill the dragon outside of the cathedral. This dragon is easy to find, it sleeps on the road that leads straight to the cathedral so you kind of have to come across the dragon in order to get to the cathedral in the first place.

You need a faithbuild to wield it. Some arcane is necessary as well. (23 FAITH, 15 ARCANE, (optional – 20 MIND to shoot enough in one go)) If you don’t have a faithbuild then perhaps a friend of yours does. If not, you could temporarily respec just for this boss. But that’s up to you.
I also suggest that you only keep one or two charges of crimson tears to restore hp if something goes wrong and then have as many cerulean tears as possible to refill focus points in case you miss your attacks.

At the start of the fight, just run up and summon as many summons as possible, remember to roll dodge his purple arrows by rolling just before they can hit you.
However once you run up to summon the Cauldron Alexander you will notice that his summon sign is on a slight dune hill. DO NOT cross this hill until your summons have reached melee range of the boss and aggroed him. Otherwise Radhan will fire a storm of arrows at you that will most likely kill you. Keep a respectful distance in this part of the fight.

If he does shoot an arrow swarm at you, just run back to where you spawned, just keep running, the arrows will eventually stop raining towards you.
When the summons start slamming Radhan, that’s when you move in like a cheetah and give him some rot.

NOTE, you can only use Ekzyke’s decay whilst on foot.

Now, the incantation has just enough range to put you just outside his melee range, if you position yourself just outside of his melee range you should be able to hit him with the incantation without getting hit.
Simply use the incantation on him AFTER he has started one of this attacks, otherwise he might suddenly aggro on you and stomp you into the ground. If he is aggro on you then just run away until he starts throwing fits at the summons, then go back and throw some rot in his general direction as long as he is aggroed on them this should be easy.

Once you hit him with the incantation enough times there will be a brown/orange smoke coming out of his body, his health will slowly tick down, this is how you know that you got him!
At this point just kite him until he rots away. That means, just circle the boss with a good distance and keep summoning the summons. Now the rot has to be applied to him twice, once in the first phase and once in the second phase.

Lorewise this makes sense in being his weakness, considering that he is already very sick and weakened by the rot. So infecting him with even more rot finishes the job so to say.

Putting this boss out of his misery also naturally grants you the achievement. So Enjoy and spread some rot!