I've updated JumboWario's world to make this easier. Here's the file:https://www.dropbox.com/s/b8yt01cpkcpzycp/Achievements%20Imp...
The nice thing about this is that there's usually a skeleton already stuck in a pond outside the cave. There should be a bow and arrows in one of the chests in the main room.
Once you leave the room, over the right side a skeleton is usually stuck in a pond. Take your bow and arrows and move 40 blocks away. (I found it easiest to go to the far side on the left on a hill and launched from there.) Make sure it turns red so you know you hit it.
If it doesn't work, back up and try again. If there's no skeleton, this world has a bunch of other cheeves set up to go.
JumboWario's original world is in this forum: Minecraft World for All 1,000 Gamerscore