interesting that you killed the summoned ones because the one we killed Rhisas, we just had the ministrels keep mezzing the summoned adds, once they summon 6 or so they don't summon anymore unless they are killed at least that was our experience. I'll have to try and kill the adds first and see if that works better. But they do so much damage it seems much riskier to me to let them wail on people while killing the adds. I had the exact same problem with the sidhe. After we killed one, we noticed we had 3 or 4 sidhe mezed with the bows. managed to kill all the bows and 1 of the sidhe before they broke out and took down all but one of our clerics. After we got rezzed we went out mid side. Tried to takes pics for this but for some reason they didn't come out. Next time though :) Yea they drop only one item but as you said its extremely nice as are most items that come off the named. The unique drops from the sidhe and such are amazing as well. From what others have said the Grand Summoner drops 5 or 6 really good items but don't think they are 100 quality. Makes the difficulty in killing him rather pointless imo since you need so many to kill him. Hopefully Mythic will up his loot table to make it more worthwhile.
-Demi