Here's one of the more humorous achievements in the game, and it's fairly easy to unlock. To get this achievement you'll need to find the designs for two Mithril items and forge them. These items are the Mithril disco phial and either the Mithril skeleton sword or the Mithril skeleton bow.
All three objects will be found while free roaming in Middle Earth. The Mithril disco phial design is found in Minas Tirith, the Mithril skeleton sword design is found at Weathertop, and the Mithril skeleton bow design is found in Lothlorien. To reiterate, you only need ONE of the two skeletal weapons. All three cost 16 Mithril bricks apiece to forge, so save up.
Once you have gotten the phial and one of the two skeletal weapons, the fun begins. Press and hold and select fifth circle going clockwise. Find the skeleton sword or bow among your many treasures, equip it, and hit your partner with .
They'll be turned into a skeleton. Using the same process, select the phial. Upon equipping it and being in the vicinity of your partner, they will begin to dance and the achievement will unlock.