I've heard you can only do it while in gliding mode, when Pit straightens out after you stop firing for a bit. I'm not sure if that's completely true, but it seems like it's a lot easier that way.
Basically, set the difficulty to maybe 5 or so (so enemies fire a lot, but you're not overwhelmed), and try to choose a stage with a lot of Monoeyes. The big black shots are the easiest to avoid. When gliding, just edge as close to the shots as possible without hitting them, taking into account how they home in on you. You should hear a little whoosh noise if you do it right. That's how I've been doing it.
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Okay, I rolled a 14. What's that mean? Hsu
That you're a cheater. This is a 12-sided die. Chan