I got really lucky to get the wafer in the last run of the game. I managed to kill satan with the most shittiest combo of items ever, I think that basically, what saved me was the wafer.
I basically hada high damage out put but had default fire-rate with Cain. Speed was decent due to some speed up pills and range was fine. I went inside Satan's room with a 1-up, a cube of meat, halo of flies and maybe sister maggy to aid me, I don't really remember. The thing is taht I had to die once to make a strategy to efficiently kill Satan in his second form without taking much damage. I think that I was really by the ropes when I killed satan as dodging the exploding leeches was really hard.