Radagon is highly resistant to all ailments and slash attacks. Get yourself a Haligtree Knight Greatshield with sacred infusion, some shield grease (or scholars shield spell), and a poke weapon (thrusting sword or spear), and laugh as he deals zero damage and just pokepokepoke your way to victory. My favorite spear is the Mogwyn Sacred Spear (Mogh's Trident) since he is neutral to fire damage, and its the longest 'great spear' in the game.

Wow, awesome!