As the card art suggests, the main use of Shields Down is to stop Obyn Greenfoot's shield scaling from becoming overwhelmingly large. It's a counter to full Obyn Control decks.
As an Obyn Control player, use Corvus Academy of Arts if the opponent uses Shields Down to break your Obyn's shields. In a pinch, this can be used to prevent OTK combos from being performed.
Shields Down is also a counter to Draining Bloon, and more specifically the Draining Bloon + Double Trouble combos that pile up excessive shield.
It's not usually necessary to equip more than one of this card, as this is a niche card that specifically is equipped with the incentive of beating Obyn or Draining Bloon, but typically not both.
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This article or section needs more screenshots. See the screenshot guidelines for more information on what screenshots are needed and how to add them. The editor that added this notice elaborates: Obyn with a shield value ending in 1, and also a shield vs no shield comparison (don't copypaste that reddit post but this post gives an idea of what to showcase
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