In a public place (like a shopping center) yell at oncoming people "YOU SHALL NOT PASS!" just like Gandalf in Lord of The Rings. For any of you who need a good reference: http://www.youtube.com/watch?v=Z1cT_5-4PJU
For those that don't know, 'pantsing' is pulling down someones pants when they don't expect it. For this prank, the more people that see this, the better obviously. You can also tabletop your friend after for a little extra flare.