This little citizen rewards careful planning by keeping your hand full
Firing it off at instant speed is also important, especially for targeted removal
Since your deck will be full of instants and sorceries anyway, that prowess ability can grow it into a 4/5, 5/6, or something waaay bigger depending on how many cards you can play in a turn
And, its so much fun that Im sorely tempted to make the changes myself
Learning From the Past Funny enough, this whole rules debacle could be solved easily by issuing an errata to Selvala, writing at the end of her ability: Activate this ability only any time you could cast an instant. This prevents the ability from being activated during the spellcasting process, as you cant cast instants during the process of casting another spell
, : Target creature can't be blocked this turn