You have at least seven cards in your graveyard and/or exile that's an instant or sorcery, or that has an Adeventure.
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.
Resolve the Brinelin trigger, returning Thrasta from the battlefield to your hand.
Resolve the 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.