You have at least six other cards in your graveyard, plus an additional four cards if
Brain Freeze is in your graveyard.
Steps
Activate
Lotus Petal by tapping and sacrificing it, adding ({U} magic symbol) .
Cast
Lotus Petal from your graveyard for its escape cost by exiling three cards besides
Brain Freeze from your graveyard.
Repeat from step 1 one additional time.
Cast
Brain Freeze either from your hand by paying ({1} magic symbol) ({U} magic symbol) , or from your graveyard for its escape cost by paying ({1} magic symbol) ({U} magic symbol) and exiling three cards from your graveyard.
Brain Freeze's storm ability triggers, creating at least two copies of
Brain Freeze targeting yourself, and any remaining copies targeting an opponent.
Resolve each
Brain Freeze, causing yourself to mill nine cards and, if applicable, an opponent to mill three cards for each