Kos isn't "easy".
Dragon Rider is easy, he could 1 shot you, and he'd still be easy, because he has like 3 moves and they're all very slow and predictable.
Besides, damage output being what it is, it's a standard for most of their bosses, even Owl in Sekiro would be a simple boss (he has like 5 attacks) if it wasn't that every attack does 70% damage to health/posture.
But that makes it not "easy", just simple.