[LDiCesare]

I found out it depends a lot on the giants. Niefel giants fall easily to fire, so fire 9 bless T'ien Ch'i sacreds backed with Fire Demons can tear them down cost effectively. This doesn't work against fire resistant Hinnom and may not be that effectie against the Fomoré.
A death bless can also cripple giants as none of them has recuperation.
As Lanka, you should probably try to resort to thunderstrike and skeleton spam.