No player will get priority in between the creature entering the battlefield and being sacrificed
You pretty much can only use Krovikan Mist with illusion-focused decks, but it should give you decent value there
Whenever The Unbeatable Squirrel Girl enters or attacks, create a 1/1 green Squirrel creature token
However, for Brainstorm to truly shine, you need some kind of combination of cards for it to be really good
As another example, if my opening hand has a one-drop, a two-drop, a three-drop and four different four-drops, then I will usually bottom the worst three four-drops in the hope of drawing into a powerful five or six-drop, while keeping the cards that will ensure a perfect 1-2-3-4 curve-out
Is Eminence an Activated Ability