Well the trouble is their MR is so high that by the time a significant number of them were blind, I'd be dead. I think the probability of it working is maybe 14%? So blinding 40 giants would take a very long time.
Abysia can make penetration items, and Blindness needs only 1 fire to cast, but you would need lots of mages for it to work. Lots of mages casting Incinerate might work too, together with lots of heavy infantry with battleaxes. Lots lots.
If you are using blood magic it has spells that would be effective, though they are high research.
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