The size of the second spell you cast determines how big of a spell you can steal, so you should look to cast spells that are free or reduce their mana cost, like Force of Will and Dig Through Time
If you get better information, please, reach out to me in the comments, on Discord, or Twitter
Red decks often force opponents to block in order to maintain their life total, so stacking first strike onto your attackers is one of the best things you can do
This can allow Bruna to become a massive threat seemingly out of nowhere, recurring potent Auras from the graveyard and allowing them to be played from one's hand without paying their costs
What if I told you that you could go even further
Thanks to only costing four mana, V.A.T.S