Spend only black mana on X.
Consume Spirit deals X damage to any target and you gain X life.
10/1/2009 The amount of life you gain is equal to the number chosen for X, not the amount of damage Consume Spirit deals (in case some of it is prevented).
10/1/2009 If the targeted permanent or player is an illegal target by the time Consume Spirit would resolve, the entire spell doesn’t resolve. You won’t gain any life.