With delirium, Unholy Heat is a clean, cheap answer to Fanatic of Rhonas and Disciple of Freyalise, their key mana engine and value creatures, before their giant Eldrazi threats come down. Combat tricks like Mutagenic Growth are largely irrelevant since their creatures like Emrakul, the Promised End and Ulamog, the Ceaseless Hunger are far outside combat-trick range, so real removal is much better here. Speed is critical since they eventually overwhelm you with giant Eldrazi, so keep your low curve intact.

Unholy Heat kills Ragavan, Guide of Souls, Ocelot Pride, and Ajani, Nacatl Pariah cleanly for one mana, hitting their entire early-game engine before it snowballs into value and tokens. This is a mirror-ish aggro race, so Mutagenic Growth is the correct cut since their deck also plays efficient removal (Galvanic Discharge, Lightning Bolt) that punishes overcommitting resources to combat math rather than board control. Keep the rest of your proactive shell largely intact, since this matchup comes down to who curves out better and answers the opponent's key early threats first. Watch out for their sideboarded Blood Moon, which can be devastating to your fetchland-heavy manabase.

Meltdown is a genuine blowout against Affinity's artifact-heavy board, capable of wiping their Mox Opal, Mishra's Bauble, Drone tokens from Pinnacle Emissary, and Munitions tokens from Weapons Manufacturing at X=0, or scaling up to catch Kappa Cannoneer. Tormod's Crypt does not disrupt their Urza's Saga activations, but it denies their graveyard value and can proactively exile their graveyard before they use Emry, Lurker of the Loch to rebuy artifacts. Mutagenic Growth is cut because their removal (Engineered Explosives) can wipe your small creatures regardless of pump, and you'd rather have sweeper effects against their go-wide artifact board.