Revived

...I think I need to buff the monk hero, (increase his spellcasting skill so that his "Holy Word" deals more damage...

The monk already deals decent damage with "Holy Word" (he could wipe out an entire army with it), but he's not very bright – he casts "Surveillance," then "Prayer," then "Blessing." After these three turns, he's left without an army, and only then, when the angel revives the remaining monks, does he try to cast "Holy Word" (if he has time) :)
There's no difference whether you attack him on the 8th day or the 13th – the hero won't get any stronger, as all those who could be defeated have already been defeated, and Nicole only has to collect the loot.