The final boss is actually two bosses back-to-back with no rest. Radagon is weak to fire and bleed; Elden Beast is immune to bleed and resists holy. You need a build that's effective against both — or be ready to adapt mid-fight.
Recommended Builds
#1: Blasphemous Blade
Fire damage destroys Radagon, and Taker's Flames has enough range for the Elden Beast.
Why it works: Fire is Radagon's biggest weakness. Taker's Flames can be spammed for massive fire damage. Against Elden Beast, the physical component of the weapon still works, and the HP-on-hit sustains you through the long fight.
#2: Frost Mage
Frost works on BOTH bosses — one of the few damage types equally effective against the entire final fight.
Why it works: Frost proc reduces absorption on both Radagon and Elden Beast. This is rare — most builds are optimized for one but struggle against the other. Frost covers both.
#3: Moonveil Magic
Versatile hybrid that handles both bosses with neutral magic damage and physical Transient Moonlight.
Why it works: Magic damage is neutral against both, Transient Moonlight staggers Radagon, and the katana's speed lets you punish Elden Beast's slow recovery animations.