Is the answer the "Right Revenge" attack used by one of Ruby WEAPON's tentacles? I think it takes off a percentage of maximum health, as well as possibly causing the 'Mini" status effect.

My Reasoning:

A sudden journey to Brobdingnag - Having been suddenly afflicted with Mini in the presence of Ruby Weapon, you would indeed feel as though you were in the land of giants.
Brought on by dexter vengeance - It's a while since I fought Ruby Weapon, but I'm pretty sure "Right Revenge" is used as a counterattack. The name certainly suggests it.
I must use my mind to survive - As Mini reduces your attack damage to 1, physical attacks are certainly out of the question for the duration of the status effect.
Lest saved by the horn of plenty - Mini is cured by the Cornucopia item, which is a mythical Greek horn of plenty.