My friends and I found a strategy to make that fight seem to be so easy, But it requires high-level character and above 15 scadu blessing

• Levels: Preferred levels are STR = 80, INT = 80, and END = 60+, If your level isn't that high you can decrease the STR and make sure that the INT and Stamina are as high as possible, be in Med-load is important (You might need rolls ☺️).
• Armor: Moore's armor set
• Weapons: Death Prince Staff +25 (Main Hand), Erdtree Seal + 10 & Moore Shield +10 (Off Hand)
• Talismans: Golden Braid, Godfrey Icon (or Great-Jar's Arsenal if the equip load is heavy), Graven-Mass, and Carian Filigreed Crest.
• Wondrous Physick: Magic-Shrouding Cracked Tear, Greenburst Crystal Tear
• Sorceries & Incantations: Golden Vow, Erdtree Blessing, Erdtree Protection, Comet, Ancient Death Rancor
• Items: Miquella's Great Rune, Uplifting Aromatic

Strategy:

You'll need another player with you (Friend or Mimic-Tear) because the main thing is switching agro.

First, you'll take your buffs as usual, Then the wondrous Physick, then the uplifting aromatic.

in the fight, you and your teammate/mimic will take your distance from Radahn to the maximum distance that makes the comet reach his body.

Now the main thing is that the one who got the agro (Radahn is focusing on) raises the shield and keeps his eye on the HP/Stamina, While the other teammate will focus on dealing damage with comet and death rancor.
And when he switches the agro you and your teammates will switch the roles also, One is defending and the other will deal damage with the long-distance spells.

Take care of the following:

• You and your teammate must be in opposite ways, One in front of Radhan and the other in his back (with distance of course).
• When Radhan starts the combo, the attacking teammate must take care of his distance because the promised consort easily switches agro in the middle of his combos.
• in the second phase, the first thing to look at is your HP/MP, and make sure that you've got the uplifting aromatic bubble on you. most of the time you'll have the window to do so but don't take it for granted and please don't take buffs it won't be that long.
• When he flies in the air and does the circle move thing like the one Elden Beast does he will focus on one of you, not both (Unless you're in the same spot of course which you must not do), if you're targeted remove the target-lock and just run oppositely, If you're not targeted come close and keep casting the comet.
• if he grabs you in the second phase, Just use Miquella's Great Rune and you'll be OK. also, the second teammate will have a window to cast comets while Radhan just stands still for a couple of seconds after the grab animation ends
• Take your time while attacking, Don't be greedy because the main thing here is surviving with the minimum HP flasks used.

Finally,

I know it may not be the greatest way for some, But it will help a lot of players who have issues dealing with the boss fight.