With Asagi, Killed 5,000 Asagi clones (cumulative).
Asagi clones are small fairy-like beings that appear in the latter half of stage 6 (they make weird sound when killed).
You'll need a lot of grinding to get this achievement since there's no magical technique to boost the number of Asagi clones appearing. Some of them appear at a place where you have to scroll the screen to right or left, so don't miss them.
It's difficult to count the exact number of clones killed, but as an example, I could count at least 210 clones defeated in one playthrough on a replay of mine (perhaps killed more).
Although I personally unlocked this on Xbox Novice, I suppose the most reasonable way to do this is on Xbox 360 or Arcade, while trying to get the Asagi no-continue achievement.
Modes able to unlock this (confirmed): Xbox Novice
Modes able to unlock this (supposed): Xbox 360, Arcade
Modes unable to unlock this (supposed): all other modes
* Changing setting in OPTION -> Game Config disables all achievements.
* Using Restart from pause menu disables all achievements in that play.Related Achievements
(Arcade / Xbox 360 1CC with Asagi)
NOTE: Irrelevant to unlocking this achievement, the achievement name refers to Asagi herself. She's seemingly cheerful, but on the other side, she's also sadistic and cruel.
[Re-posted after The Site Crash.]