Athene's method was by out of group tagging in instances, which was an exploit (that has now been fixed).

Out of group tagging in the open world is fine, however you get exp based on the damage done. I tried dropping 1 shaman from my team of 5 to tag a mob before the others, and found it was getting about 3x more experience. I'll be using this method individually dropping the level 80s from my team as i start hitting 80 (mine all have different exp, out by as far as a 70% of a level at 78 currently).

But bottom line is, tagging out of group works, but it works on how much damage your character is doing. Best option is spec 1 (or more) of your shamans resto, whack the turd shield on your deathknight, spec it unholy, and go nuts with the aoe pulls.

Not sure what you were doing wrong :P