You can use Extreme Heat to remove a burning bloon of your choice. It is best to instantly kill a high-HP or "dangerous" bloon.
Extreme Heat on a burning Volatile Bloon allows its explosion effect to trigger on both sides, no matter which side the bloon is.
With 2 charges and no possibility to return the card, Extreme Heat is better at stopping enemy bloons than Return to Sender, without any risk of giving the opponent extra sources of Bloontonium.
However, this is not to be used to remove an opponent's The Eternal, or else it'll scale further, unless the opponent's deck is already full.
Players who are seeking to boost their own The Eternal could use Extreme Heat to instantly pop it, along with Bloon Melter. To allow The Eternal to come back, always have 7 or less cards in the deck if using a 2-charge Extreme Heat.