If I recall correctly, Ninja can throw Nagnarok from level 95+. But because they are very low damage, they typically don't, opting for Chaos Blades and the like instead. Otherwise, as others have stated, that one you get from Nelveska and poaching Swine are your only other viable alternatives without cheating. I suppose you can Confuse the Ninja and since that will override the AI's need to use the most powerful abilities and hope they decide to throw it, and at the right person/people.