Spellbinding Soprano's encore ability by paying ({3} magic symbol) ({R} magic symbol) and exiling it from your graveyard, creating a token copy of it with haste for each opponent you have.
Declare each
Spellbinding Soprano token as an attacker.
Each
Spellbinding Soprano token triggers, causing instant and sorcery spells you cast this turn to cost ({1} magic symbol) less to cast for each
Spellbinding Soprano token.
Cast
Sprout Swarm with buyback by paying ({1} magic symbol) and tapping a green creature you control.
Deekah triggers, creating a 0/0 Fractal creature token, then putting two +1/+1 counters on it, Resolve
Sprout Swarm, creating a 1/1 Saproling creature token and returning
Sprout Swarm to your hand.
Cast
Sprout Swarm with buyback by tapping the Saporling and Fractal tokens.