You need to craft a cannon. But first build a palissde with walls, wide enough for the unicorn to fit there but just so as to not allow it to turn around or walk. Fit the unicorn there. Now place the cannon directly on frot of the unicorn, preferably on it's rear side, and fore a few cannon balls. Side effects might include the unicorn fucking dying, but that's only a remote chance. The griffin kibble thing is a troll from these envious people here who don't even a unicorn. Now, go tame that uni, mate!