If you have the strength spell, you can try using it against the cobra by talking to him. Or if you have the focus stone equipped or the magic wand you can use the spell that way. After using the spell you can come back for more
Have you used the strength spell and returned to the shop? You should be able to get it. Also if you beat the Cobra, you'll get the magic wand that'll let you cast spells as long as you have the arousal level to cast them
