Maximum Bert said:
I think there was one in FFIV that copied your every attack only with much more power so he basically killed you every time you hit him or wiped you out if you used an aoe attack on him. The trick to beating him easily was simply do absolutely nothing and you win after a few rounds. It wasnt immediately obvious that this is what his weakness was but fairly logical took me a little while to figure it out more of a puzzle boss I suppose but I enjoyed it, guess while its unexplained it is foreshadowing the solution.
Are you sure that was FF4? Sounds like the Gogo fight to get Mimic Job in FF5.
In Dragons Dogma, the titular Dragon is weak to Dark Element, and strong against Holy (which was the one you probably equipped for the dungeon of demons immediately leading up to him, and the generally most consistent element to use).
This isn't actually explained or hinted anywhere that I've found. If you have a pawn thats seen it happen, they'll yell it out though.