I watched the first three episodes. Interesting fights, but they would have been more balanced (and more historically accurate) like this IMO:
Apache vs. Ninja
Gladiator vs. Spartan
Viking vs. Knight
As a secondary option Gladiator - Knight and Viking - Spartan (but that's historically anachronistic, unlike the above)
Pirate vs. Samurai - don't really like that one, but can't think of who to put the Pirate against. And Samurai vs. Viking was realistic historically (I mean from a time period point of view). Samurai vs. Knight would have also been interesting. Though I admit that there were historical periods when samurais and pirates co-existed.
I wonder how the show runners selected the fights.