I wonder how quickly it multiplies? If you're technical, you can't kill all of them in the same instant with one sword swing, it would just be a very small collection of instants that are collectively too fast for the rabbit to multiply after.
On that note, I wonder what tier of fighter can kill the Ousagi by themselves.
Reinhard and Reid, probably.
Cecils, maybe.
Prime Wilhelm, maybe.
Julius seems possible if he scales to Cecils who scales to Reinhard.
Obviously Prime Ram.
Etc.