Shulkers are able to deflect arrows when closed. This is a feature, but if you fire a low-power arrow above them, it may rapidly deflect until they open again. While experimenting with this bug, I also found that tipped arrows still have their effect, even when the Shulker deflected them. Combining these two bugs with Harming II arrows can kill the poor Shulker in about a second, because every time the arrow deflects and hits again, the Harming effect is reapplied. This was on the PS4 edition. I could not take any screenshots.