The Blessing of Light doesn't work against mortals. The WoL's effectiveness is greatly increased against 'divine' level enemies because of the blessing. However without it s/he is just a very skilled and competent fighter. Plus I suspect Zenos himself has a few power ups. Back when Stormblood was announced Yoshi P said we would learn his motivations and how to fight him. That suggest walking up and just punching him in the face might not be particularly effective.
If you remember the Ultima fight, our attacks did almost nothing before the Blessing of Light kicked in, at which point we started ripping the machine apart. Likewise, we survived its ultimate because of the blessing. Even when we fought Ravana and Ifrit we had a Crystal of Light active. The Blessing of Light is our sword and shield against primals and the like but its not really much vs. mortals. Against mortals we have to rely on our base strength which, while formidable, isn't unbeatable.
