Well, I managed to best The Ancients! In the end, I didn't use either my merc or any summons. It was just me and gumby up there alone.
Here was the strategy I used: as soon as I touched the altar, I cast decrep on all three. Then I ran away through two of the pillars. When they came at me, I recast gumby between the two pillars to keep them occupied. Then I used lower resist with p. dagger and/or nova. Whenever they broke down gumby or wandered past him, I ran off to a new position. Rinse and repeat.
Perhaps not the most melee approach, but it got the job done. (Actually, it almost got the job done twice. I once had two Ancients down and only the leaper to go, but he got me with a one-hit kill when he had ~10% HP left. :rant: )
Many thanks to everyone for all the suggestions! :thumbsup: