Osakna of the old ones

whenever Osakna or another elder enters the battlefield under your control look at the top six
cards of your library. you may exile an instant,
sorcery or enchantment card from among them
and put the rest on the bottom of your library in
a random order. you may cast the exiled card for
as long as it remains exiled.
elder creatures you control get +1/+1 for each
card exiled with Osakna.