It looks like she is standing upon it, not slaying it.

spoosedly there is a small about 3 inch or so triangular area between a bulls neck and shoulders where bullfighters try to stab withtheir spears/ knives (I donot knowthe spainish word for the specific weapon) . a short cut will damage the nreves and muscles of the bull making it more difficult fot the bull to turn its head /gore withits horns . a deep cut wil sever the spinal colum causeing death / parlyzation .

Durga is standing on Buffalo from where has emerged the 'Asur' the monster, She is holding his head with left hand to chop it off with chopper bladein right hand. Beautiful 9th century sculpture on stone from Java