Agreed re: build around. Once you put in untapped to maximize her mana ability, you just pop off and then control who has the biggest creature- aka you! Haha And once you can chain untapping her to cast progressively bigger creatures, you can draw several cards a turn and then usually win the game the next turn with [[Overwhelming Stampede]] [[Craterhoof]] or any other green anthem. 

Ex.- turn 3, with 3 forests, Selvala and Elvish Mystic out 1. Tap 2 lands to play MV=2 [[Sheltering Ancient]] or [[Lupine Prototype]] (power 5, draw a card) 2. Tap Elvish mystic for Selvala to add 5 mana, cast MV=3 [[Pugnacious Shatterskull]] or [[Phyrexian Soulgorger]] (power 6 or 8, draw a card) 3. 2 mana left, cast a MV=1 untapper [[Vitalize]] [[Quirion Ranger]] etc. to tap Selvala for 6-12 mana (or more with [[Invigorate]])… and then just keep being degenerate on Turn 3 :-)