Dwarves versus Orcs.

But I would choose Light; it will definitely help in reducing enemy morale by simply increasing the damage dealt by those who are good at it: your own troops.

...and the later the stage, the more relevant this becomes. Mass Haste, Divine Shield, Stone Skin, and Righteous Strike, combined with Attack and Defense Aura, plus proper troop positioning, will create an impenetrable horde of shorties, and runes too...the Orc will have a hard time.

Added 4 minutes later

immediately eliminate his centaurs so that he doesn't stand still and starts attacking, and rely on "Readiness" yourself.
or is it better to upgrade Battle to the Last, with Frenzy and Archery in attack, and then go on the offensive?

Centaurs are perfectly neutralized by Mass Evasion (if only the guild allowed it), and standing still in a defensive formation is an option if you upgrade your troops with Mass Stone Skin and Righteous Strike. The downside is that tightly packed troops are very desirable targets for enraged Cyclops