You control at least two untapped creatures, one of which must be green.
({3} magic symbol) ({R} magic symbol) available.
Steps
Activate 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 tapping a green creature and any other creature you control, creating a 1/1 Squirrel and a 1/1 Saproling creature token and returning Sprout Swarm to your hand.