As a shaman, group up with your friends and go to an area where guards happen to be spellcasters, i.e. the Transitus Shield in Borean Tundra.
Drop your grounding totem, (NOTE: you must NOT have the grounding totem glyph for this to work!) and have your party members go wild and attack everybody in sight. The guards will retaliate and their spells will hit your totem.
As long as you the shaman, don't aggro the guards, the spells harmlessly hit your totem. No damage will be done to your totem and your totem will stay up for the full duration, essentially making your party members immune to the guards until they run out of mana.
Keep refreshing your totem and you can kill without worrying about guards. Keep an escape route in mind and drop grounding totems to cover your friend's escape.
Figured this out when WotLK came out, just retested it with the Cata updates. Worked earlier today on US - Stormscale.