Ment
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I also noticed that doubling the coordinates sometimes works, but not always. It seems impossible with flying arrows and fireballs. I haven't even seen the developers do that.
How to do it: here's a screenshot from the developers' video description. You can see it there.
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As for the wall: I don't think it's necessary. It will play until you're blue in the face, if I'm not mistaken. And then you need to add an effect to make the wall disappear. But I can check.
