Zenos actually is Lv70 to begin with (which makes the inevitable losses to him even harder - it takes a lot of tries just to get that sliver of damage in), but as you proceed through the story you do get stronger. In the first fight you just endure his blows until Zenos' cheap Garlean katana breaks (katana are not known for durability) and he gets bored and leaves, dismissing you as unworthy of his time; in the final battle he's going at you full tilt with a legendary weapon (Ame-no-Habakiri) and you fight him to a standstill or maybe even decisively prevail (he's winded after the fight; the Warrior of Light is not).

... though it helps that in the final showdown the Warrior of Light has three others backing them up. Either way, Zenos canonically acknowledges your growth in performance.

Shinryu? Ah, well, the Warrior of Light specializes in killing eikons... so that was not the smartest move. (They also had seven people backing them up, so...)