Instructions
Travel to an area on the map marked with a new portal to meet the shadow beasts. They should fall from the sky and surround you with an electric fence.
Kill all the beasts until you have only two left. Most of the time, there are only three beasts. However, occasionally, there are four beasts. Kill each beast individually until two living beasts remain.
Wait until the beasts come very close to you before doing a spin attack (by shaking the Wii remote); attacking both of them simultaneously. If they don't die from the first spin attack, repeat this step until they die at the same time. You must kill the two remaining beasts simultaneously in order to defeat them -- otherwise, they'll all revive and you'll have to start over.