Once your graveyard is stocked with seven or more cards, each attack exiles one at random, and you get the chance to play it that turn
While the "ease of flipping" category is, in large part, a product of the number of defense counters on the battle (battles range from three to seven defense counters, and a battle with three defense counters will be much easier to flip than one with seven), it also takes other factors into consideration, like whether the battle makes creatures (if so, we can use them to help defeat the battle, making it easier to flip) or whether the battle acts as removal (which can get a blocker out of the way, making it easier to attack and defeat the battle)
Ultimately though, the age, rarity, declining population of raw variants and sheer nostalgic value make buying 1st Edition WOTC Pokemon cards on eBay a great place to start
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