Allowing a character to be exerted puts that card at potential risk on your opponents turn
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There are actually 309 cards in total, and each Commander deck contains 25 cards that haven't been seen in Magic: The Gathering before
If you are capable of healing your cards, through items, actions, or character abilities, and Beast, Tragic Hero has damage on them at the start of your turn, you can let them attack and then heal them, allowing you to still benefit from the increased strength but keeping them healthy enough to stick around
You need to be careful with your triggers on this one, as you want to make sure that the Cactuars 10,000 Needles resolves before the Mage Slayers does, otherwise, youre just dealing one measly point of damage