Another helpfull hint for that annoying perfect achievement.
Completing the final mission on expert. (As I got a gem and this item I'm guessing it's just completing the secondary objectives on expert) The item you gain from this is "Bracers of Jason, all attacks are perfect attacks.
Equip them and go into rampage. Find a group and hit X six times and you get your achievment. Sure beats trying to time those swings!