Mardu Energy floods the board with cheap creatures and generates value through Guide of Souls, Ocelot Pride, and Ragavan, so Thing in the Ice is your most powerful sideboard tool — it resets their entire board when it flips while leaving you with a 7/8. Boomerang Basics does nothing relevant here since you're not attacking their mana base, and Mutagenic Growth loses much of its value against Goblin Bombardment, which lets them sacrifice creatures in response to blank the pump. Subtlety provides free tempo against their key early threats like Ragavan and Ajani, Nacatl Pariah without spending a card, and Brazen Borrower can bounce a problematic permanent like Goblin Bombardment or a token-generating planeswalker to buy critical turns. Post-board they will bring in Thoughtseize to strip your key pieces and Wrath of the Skies, so prioritize pressuring them before disruption lands and keep your cantrip density high to find Thing in the Ice quickly.