found this today so ill copy/paste here
(credit goes to Trois for typeing but im sure he got it somewhere else)
"have your tank run up to the rune guardian and absorb the first att, in that time, the rest of you should run right under him without doing anything else. Now, the tank will be taking damage consistently from his attacks, but the rest of you should be relatively safe except for a weaker aoe attack (with 1-2 priests group healing this is nothing) i think (don't fully remember what hits ya, but its not his one-shotter), so just make sure you're under him at ALL times (a guideline to follow is to select the guardian as target, and have your chars stay inside his red circle) and you move with him if/when he moves. It's a very easy battle if you keep this up. "