Which enemy drops the sealed nodachi?
Any mob in Cradle should be able to drop it. Both of my swords came from random trash mobs. PSE bursts really help your chances since every mob that dies is another shot at the sword.
Does the quest need to played at ultra hard?
I believe so. I don't know if lower difficulties can drop 15* items.
Lastly as a word of advice, if you really want the sword you NEED to stack as many boosts as possible. I was only able to get my first drop after grinding cradle for about a week straight while there was a 300% RDR boost from one of the campaigns and using the best boosters I had.
Your best shot will be to stack the best boosters you have with the boost from PSO2 day (the 21st if you have premium, 7/1 otherwise).