You have at least seven cards in your graveyard and/or in exile that are instants, sorceries or have an Adventure.
You have a storm count of 3 or higher.
Steps
Activate
Gwenna by tapping it, adding ({U} magic symbol) ({U} magic symbol) that can only be spent to cast creature spells or to activate abilities of creatures.
Gwenna trigger, putting a +1/+1 counter on it and untapping it.
Activate
Gwenna by tapping it, adding ({G} magic symbol) ({G} magic symbol) that can only be spent to cast creature spells or to activate abilities of creatures.
Cast
Thrasta by paying ({G} magic symbol) ({G} magic symbol) .
Brinelin and
Gwenna trigger.
Resolve the
Brinelin trigger, returning
Sailors' Bane from the battlefield to your hand.
Resolve the
Gwenna trigger, putting a +1/+1 counter on it and untapping it.
Repeat.
Notes
You can start the combo at lower storm counts if you can pay for
Thrasta until your storm count reaches 4 when you cast it.