Knight of Owner only makes D Rank Phantasms, which can't even pierce Siegfried's skin so with only A Rank Strength Lancelot isn't accomplishing much, and Arondight's Dragon Slaying doesn't actually negate Armor of Fafnir, just prevents it from raising its defense to B+ for blocking Arondight, so even that wouldn't do too much damage, and the sheer drain of energy Arondight brings is crippling. Drawing Arondight is ultimately what killed Kariya pretty quickly afterall.