You need to activate magic in the same frame that a shuriken is in the boss's hitbox. I haven't gotten it to work when buffering the magic on the way down from a jump. I press the button when on the ground, and if a shuriken is in the spot , it should work. I have a method for timing this, but I've seen others just spam stars to increase the chance of coinciding with the magic.
Yeah, ditto what z_tox said, especially about activating magic before you land. If you can throw an extra shuriken into its hitbox, it will allow you a bigger window to trigger the glitch. So, spamming them seems like a good approach. Doing it with just one shot can be quite difficult.
In general, you probably want to adjust your distance to the boss to increase your odds. If you watch other runners, you may be able to tell by where they're standing and how they're jumping when they get it. Note, the head can spawn at 3 different heights (though it may be difficult to tell quickly), so the optimal approach can change depending on which height the head is at.
