• all random arrays of monsters are empty
• the task doesn't really make sense (the way the swords are taking damages: not logical at all in the given context) => it's very/too much artificial

actually...

a wrong algo can pass all the tests (not even using the correct function)

Your kata is a combination of these 2 or more which make it a duplicate nontheless:

1 && 2

Note to author: please don't resolve this issue!

It is theoretically possible for several unidentical swords to take the least damage for slaying all given monsters. A tiebreaker should be specified.

( Yes, I know you have finite sets of swords and monsters for which this problem probably does not arise now. Still, either set might change in the future, and the problem might arise then. )