A rainbow deck with a decent number of shield spells (bone, phase, and possibly a dissipation), miracle, heal, and some empathic embraces seems to get ~75% mastery, 90% win and 10% crappy draws resulting in death. My strategy is to throw out a shield asap, and then put out creatures but keep them weak until either I have miracle and heal (reset to 100) or lots of creatures and an empathic embrace.
It's weak to momentumed creatures, though, and potentially to poison, although I've tended to find that against poison-wielding elementals, I lose out on the mastery much of the time, but I rarely lose.
