As long as Fleur is prepared, you don’t lose unspent mana as steps and phases end.

Whenever you attack, add {R}{R}. Then you may have Fleur deal damage to you equal to her power. If you do, she becomes prepared.

4/2

Hearth Fire{2}{R}

Sorcery

Exile the top two cards of your library. Until the end of your next turn, you may play those cards.