At first tainted Fiend seemed difficult, but after learning a few things about him he wasn't so bad.

You don't take damage on curse rooms when you're small
If you're at half a heart, going small can get you many extra hits. When you go small, you get two hits before dying, simply go big when one of your fiends die. Switching back will replenish your extra mini fiend. I managed to switch back to big fiend after my last mini fiend died once, but couldn't recreate it.
When reforming into the large fiend, you can't take damage, which can be used defensively.
When you're small, you can pass under tears, depending on the tears' height, like pluto.
When you're small, your frames reduce significantly, making it easier to dodge.
When you're small, you place multiple tiny bombs, one for each mini fiend. This works with bomb items, like ghost bombs, which can easily shred bosses.