Unlocked this with Kurisu, while on one of the first few Arcade stages, wasn't even trying for it. This leads me to believe that you'll have a much easier time using her, if you're trying for the achievement. Hope this helps someone
Got this with Yuzuha without trying while on the third stage when some enemies jump from the buses in the background. It's a bit after you beat the sections where you have to run left to proceed and are introduced to the big white enemies. Pretty sure it had something to do with having full energy and using the RB attack that shoots orbs everywhere. It'll hit everyone and do a lot of hits each. I'm assuming that's how it accumulated so easily.
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