Nariko is facing a whole squadron of Roach's pets. A little while after you start the battle Kai will throw the heavenly sword to you.
Once you have the heavenly sword the battle will be quite a bit easier.
If you are good at reposting, repost these pets to take them out quickly.
Otherwise run circles around them until you find one pet that is somewhat alone. Single it out and beat it down.
When it falls on the ground be sure to stick your swords in it to finish it off. Remember that any red attacks they throw at you are unblockable, so roll out of the way.