You're all making this combat far too difficult.

I put a character who was useless in this combat (rogue because not enough +3 or better arrows) at the bottom of the stairs from the platform and cast Otiluke's Resilient Sphere on her. The dragon couldn't get to me and my characters pelted it with missle weapons while summoned creatures got up close and personal.
This was with high lvl characters in HOF, so I suppose summoned critters might not be able to damage the guardian if lower lvl characters summoned them.
I thought about using one of my undead as the Otiluke's Sphere blockade but I was afraid it might resist the spell or something.