Warcraft: Arthas Menethil vs Kel'thuzad

• Arthas has composite feats. (Excluding: Lich King feats) within the canon Warcraft universe.
• Arthas is in-character.
• Arthas has in-universe knowledge on Kel'thuzad.
• Arthas has Frostmourne.
• Win by death.

• Kel'thuzad has composite feats within the Warcraft universe.
• Kel'thuzad is in-character.
• Kel'thuzad has in-universe knowledge on Arthas.
• Kel'thuzad has standard weaponry.
• Win by death.

Bonus Round: Same conditions, but Arthas gets LK feats + Kel'thuzad gets the Book of Medivh with a hour of prep to use it in advance.

Location: The Frozen Throne

I think Kel'Thuzad would likely win because he knew vastly more about Arthas's own powers than Arthas himself did. In fact, everything Death Knight Arthas knew was because KT told him to begin with. His knowledge about the Lich King, Frostmourne, and necromancy surpassed that of the dreadlords, the very beings who aided in the Lich King's creation and oversaw him. Given this, I'm relatively confident he could devise a way to defeat him.

arthas was already stronger than kelthuzad as paladin and killed him, after he became death knight and kel thuzard a lich he was still superior and above him, no need to mention the later scalings as he is consistently above even more