Mmmh maybe Mass Darkvision, at 50%, should be a nice thing, but probably darkness is wanted to be not so easy uncounterable so let's just forget this
As you have many mages, maybe you could start a mind hunt - seeking arrow spam in order to kill many of their casting mages as possible, possibly right before an attack. Can fliers fly in darkness? If so, hold and attack rearmost could be great, expecially for a thug with winged shoes and some aoe weapon... or you should just focus on something else, like heavy guerrilla and raidings to ruin his economy and make their big armies start diserting, and take time while those big armies go around trying to recapture lost provinces
P.S. well if solar brilliance blinds everybody, at least the advantage of the enemy is zeroed no?