You did the Devil's Snare in your first class operating under Salazar's Rules, didn't you?
Sally-Anne is right; Professor Desai is clever. Perhaps it's not so much that you need to convince her that you're pants at Defence. If she talks with the other Professors (such as Professor Sprout) she might discover that you're really brilliant at Herbology, so she might wonder why you can't do well in her class. But you might be able to play it as if you really are obsessed with Herbology, so you stick to Herbology-based defences.
Of course, you can't conjure mandrakes and lob them at your classmates. Madam Pomfrey would be very upset, wouldn't she? But there are other aggressive plants you could conjure, like venomous tentacula. Or you could try the Sleeping Beauty spell.
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Sally-Anne is right; Professor Desai is clever. Perhaps it's not so much that you need to convince her that you're pants at Defence. If she talks with the other Professors (such as Professor Sprout) she might discover that you're really brilliant at Herbology, so she might wonder why you can't do well in her class. But you might be able to play it as if you really are obsessed with Herbology, so you stick to Herbology-based defences.
Of course, you can't conjure mandrakes and lob them at your classmates. Madam Pomfrey would be very upset, wouldn't she? But there are other aggressive plants you could conjure, like venomous tentacula. Or you could try the Sleeping Beauty spell.