I decided to make a tutorial because @zzzergling mentioned that the magic wands from the badges don't work in a multiplayer setting.
Luckily, you CAN make magic wands that work with friends -- i.e. for wizard duels. The code is just a bit more complex. So I figured I would do a quick tutorial for advanced users (where "advanced" means you've done/understand the majority of the "Learning to Program" badges, especially the "Events" badges).
Now just a couple of tweaks to the "lavaball" and "explode" functions...
This is basically the same as what's in the Explosion Wand badge except that it adds some metadata to the falling block entity. This will get checked later in the "explode" function to see who launched the falling block.
Again, pretty much the same as the badge. But this function checks who launched the block by looking at its metadata (which was set in the "lavaball" function). It then creates the explosion if the person who launched the block is holding an Explosion Wand.