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Together, this led to the character getting both the Leviathan and Guppy transformations, about 15 damage in body mode, and a wholly-unexpected combo:
Using the soul mode on the Forgotten, I had a supply of six black hearts (which had a tendency to drop from Abaddon's skill). Due to the wafer, I never took more than 1/2 heart of damage...and when I did, Athame and Celtic Cross would both proc. The black ring of death was very high damage and could run through enemies, which would then have a chance of dropping a black heart to replenish my health.
With this combo (and in soul mode!), I was able to take down both Greed and Ultra Greed, with about three black hearts left at the end. My only regrets were not capturing the run, or writing down the seed (didn't occur to me until I was relaying this event to my son, long after I had closed it).