I've always wanted to play that game, but haven't had the ability to do so because i have neither a Japanese PS2 nor a swap disc for my PS2. That's going to change at the Midwest Gaming Classic!
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Also, somehow that dude manages to beat it without getting hit. He seriously must have been playing the game at 1/20th the speed if that in order to pull that off.
I might have seen him get hit once or twice, I can't tell due to the horrible resolution of that video. It is quite a feat to beat that boss without using a continue, but it certainly is possible for two reasons:
1: Your ship's hitbox is
TINY. As I recall, it's a few pixels big in Mushihimesama. This means that in order to harm you, a bullet must hit the tiny area.
2: The bullet patterns are set and don't change every time you play. It is possible to navigate the bullets and avoid them, but it takes a ton of trial and error.
I'm not trying to take anything away from the fact that he beat that game, I'm just saying that it is certainly possible to do so if you're willing to work hard at it.
That just doesn't look very fun.
It looks really fun. That's why I love shmups like this one.
Here's another fun bullet hell boss: