I just summoned a lot of dead people and hade my mages summon new when they died. My cleric did cast som protecting spells, recitation, bless etc.
Most importent however,improved haste; makes the kiling go faster..
Lastly my fighters whacked the Guardian from behind. Oh, and just cast improved invisibility on the character that summons the Guardian. The Guardian will ignore that guy.
Killed him on the first try, so easy.

Need not too at all.. Just cast out some creature to hold him off then use your wizard - high level the better to cast out mordenkain missiles at least 3 times and the dragon will drop like a dead duck without any of your party members hurt in any way