There’s only one way you can make sure that Elsa gets control of the Fruitcake Elemental and can’t give it back: Cut off her access to mana. The problem is that Elsa can simply add the mana to her mana pool in response, so the best thing to do would be to perform all your actions in one go, and then let the turn structure empty your mana pools for you.

1. Tap two Jungle Shrines, three mountains, a forest, and a plains to add (one red mana and all white mana to be used as generic mana) to your mana pool.
2. Spend to cast Punishing Fire on Elsa. Elsa may tap the Reinforced Bulwark to prevent 1 damage to her. Elsa is now at 6 life (7 life if she used the Bulwark).
3. Because Elsa was dealt damage by a red instant or sorcery you control, you can now return Chandra’s Phoenix from your graveyard to your hand.
4. Tap Grove of the Burnwillows to add (used as generic mana) to your mana pool. Elsa gains 1 life. Elsa is now at 7 life (8 life if she used the Bulwark). Put a +1/+1 counter on Kavu Predator.
5. Elsa gained life, so you can now spend to return Punishing Fire from your graveyard to your hand.
6. Spend to cast Natural Affinity, turning all lands into 2/2 creatures until end of turn.
7. Spend and activate the Martyr of Ashes, sacrificing it and revealing both Chandra’s Phoenix and Punishing Fire from your hand. Martyr of Ashes deals 2 damage to each creature. All lands — yours and Elsa’s — die; Elsa may tap any or all of her lands for mana in response.
8. When Boros Reckoner is dealt 2 damage, choose for the Reckoner to deal 2 damage to Elsa’s Overgrown Battlement. The Battlement now has a total of 4 damage and dies; Elsa may tap it for mana in response.
9. Declare the start of your combat phase. Elsa’s mana pool empties; so does yours, but you have no mana in your mana pool at this point. Declare no attackers.
10. On your second main phase, tap your Viashino Fangtail to deal 1 damage to your Cathodion. Your Cathodion now has a total of 3 damage on it, and dies.
11. Cathodion’s ability triggers, and you add to your mana pool.
12. Spend to activate Fruitcake Elemental’s ability, giving control of it to Elsa. Because Elsa has no lands, no mana in her mana pool, and only one creature that can produce mana (the Sylvan Caryatid), she is unable to give it back to you.