"Perhaps try the stabbing instead of the slicing," Al suggested. "You won't need to cut as much, just a quick stab in a relatively vulnerable area."
Al took a peek at the anatomy book, then at the pork ribs on the table. "Let's try them next, just to get you used to the texture. But honestly, don't bother with the ribcage just to get to the heart, that's too thick. And targeting his wand arm won't help you if he casts wandlessly like you. But... perhaps, a few well-chosen stabs: at the neck or the spinal cord, or right in both eyes, or," he pursed his lips critically, "genitalia. Keep the slicing effort to the minimum."