Just constantly keep moving when Satan's in his third form. Once you see the second shadow you know where both hooves are, so you can stop moving and hit the leeches/first hoof. There will be a few more seconds after they're both down, then repeat. It's not really too hard, but it's frustrating if you get to him with low health as it's hard to get used to his patterns.
Also in the second phase, just stay to his sides, and only cross in front after he does a forward Brimstone.
Well the hooves are not the main problem, the kamikaze flying leeches are. And the game is so goddamn dark on the Vita I can't see what's going on most of the time.
Maybe I'll try some challenges to unlock new weapons or something.