I don't know... the first 5 weren't really that hard unless we're counting FF4 DS. I mean I found them hard in the sense that most boss battles made you feel like you were right on the edge of dying on every turn, but I can count the times that I actually died on one hand for each game in the series.. with 13 I died countless times.
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http://soundcloud.com/thundersheep/lunatic-asylum
http://soundcloud.com/thundersheep/summer-rain