>>713758425 (OP)He's hard no matter what.
But especially so if you refuse to interact with the mechanics at a higher level and just try and brute force everything.
He's fat and doesn't dodge, so you can really build up status on him, so Arcane builds in particular can melt his health bar - Bleed/Poison/Frostbite.
Great shields are well known for doing extremely well against him too. But they're just extremely good across the entire game.
While he loves to get in your face, the arena is big enough that you can really get a lot of mileage out of just running away if you're a bow or caster. Hit the bricks!
Spirit ashes are best saved for phase 2 imo, as they tend to take too much pointless damage on phase 1, which isn't that hard in the first place.
You can even basically skip phase 1 with some Rot Pots as Radahn has very little resistance to them.