Logic

When this enters the battlefield, add {w}.
every turn, check if your devotion to white is more than seven. if true, add {w}{w}{w}{e}{e}{e}.
pay {e}{e}{e}{e} with a devotion to {w}: your opponent looses 3 life.
{4}: Level up. (this starts at level 1. level up to get +1/+1, and also decrease all ability costs by 1.)
Oh, Logic always starts fights, but I prefer not to.—Mischief