Extreme Heat is good at eliminating high-health Bloons or Bloons that benefit the opponent when damaged, such as Weakening Gas Bloons and Growth Gas Bloons.
Extreme Heat pairs particularly well with It's All On Fire Now, allowing the player to set any Bloon On Fire and then destroy it with Extreme Heat immediately for only 5 Gold, and both cards have 2 charges.
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.
This can be used to remove scaling Bloons such as The Eternal or Bolstered Bloon or else it'll scale further, unless the opponent's deck is already full.
Extreme Heat is very effective versus big health Bloons such as The Regenerator, Lead Zeppelin, Golden Blimp, Soulstealer Bloon, and other powerful blimps. For this reason, Gwendolin can run this if she is struggling against tempo decks.
Keep in mind that Gwendolin's abilities are already very strong against tempo, so using Extreme Heat may just take up deckspace for something she could already do.