I have had pretty good drop rates for epics. However, I had a LONG string of NOT getting the epic with Icebloom and Yorick on the same team. Icebloom had the epic and Yorick did not. And I was not getting Yorick's epic after perhaps 12 tries. Then it occurred to me that Icebloom preceded Yorick in the lineup every time. I simply put Yorick in front of Icebloom and boom. The epic dropped on the very first run with that order. This leads me to believe that if you have two heros looking for an epic, position the one you want to get it first in the lineup. I suspect the code looks at the first eligible hero and decides if they have the epic already or not and configures the dungeon accordingly.
I actually recorded the boss fight and epic drop. As a side note, I used Astrid's Derision to pin Boss Frostbite back on Glacias and when he went for an attack, he essentially committed suicide: