If an opposing Active Pokmon has energy attached to it and is ready to attack, a Repel gets rid of it, and the opponent may not have another Pokmon with energy attached yet
What makes them happy
They look for the most dramatic, most well-documented, and most visually impressive errors regardless of age, and they pay close attention to the underlying card's standalone market value as a baseline for the error premium
You could cast a creature with flash to block your opponents attack, or even cast a creature on their end step that can win the game the following turn
Finally, all prices will be from the cheapest available printing, so long as that version is still of mythic rarity
these effects are fairly expensive and generally sorcery speed, so cards like Swan Song and Counterspell are perfect answers