Don't forget that the Warrior of Light didn't fight the ascians and primals alone. All the time quests have us form a group, in the story, to fight the primals, run dungeons, etc.

Against Zenos you're all alone. You're basically trying to solo a dungeon boss that would take the Warrior of Light, and 3 other allies to beat. (Saying this as an example, not a spoiler.)

I'm sure if Zenos went against, say, King Thordan by himself, Thordan would win. Yet we best Thordan with the help of allies.